Add nix formatter build command

`nix formatter build` is sort of like `nix build`: it builds, links, and
prints a path to the formatter program:

    $ nix formatter build
    /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt

Note that unlike `nix build`, this prints the full path to the program,
not just the store path (in the example above that would be
`/nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt`).

Motivation
----------

I maintain a vim plugin that automatically runs `nix fmt` on files on
save. Since `nix fmt` can be quite slow due to nix evaluation, I choose
to cache the `nix fmt `entrypoint. This was very awkward to do, see the
implementation for details:
7864607231/lua/null-ls/builtins/formatting/nix_flake_fmt.lua (L83-L110).

I recently discovered that my implementation was buggy (it didn't handle
flakes that expose a `formatter` package, such as nixpkgs), so I had to
rework the implementation:
https://github.com/nvimtools/none-ls.nvim/pull/272.

With the new `nix formatter build` command, I can delete all this akward
code, and it will be easier for other folks to build performant editor
integrations for `nix fmt`.

src/nix/formatter-build.md

@@ -0,0 +1,23 @@
+R""(
+
+# Description
+
+`nix formatter build` builds the formatter specified in the flake.
+
+Similar to [`nix build`](@docroot@/command-ref/new-cli/nix3-build.md),
+unless `--no-link` is specified, after a successful
+build, it creates a symlink to the store path of the formatter. This symlink is
+named `./result` by default; this can be overridden using the
+`--out-link` option.
+
+It always prints the command to standard output.
+
+# Examples
+
+* Build the formatter:
+
+  ```console
+  # nix formatter build
+  /nix/store/cb9w44vkhk2x4adfxwgdkkf5gjmm856j-treefmt/bin/treefmt
+  ```
+)""

src/nix/formatter.cc

@@ -1,6 +1,8 @@
 #include "nix/cmd/command.hh"
 #include "nix/cmd/installable-value.hh"
 #include "nix/expr/eval.hh"
+#include "nix/store/local-fs-store.hh"
+#include "nix/cmd/installable-derived-path.hh"
 #include "run.hh"
 
 using namespace nix;
@@ -25,7 +27,7 @@ struct CmdFormatter : NixMultiCommand
 
 static auto rCmdFormatter = registerCommand<CmdFormatter>("formatter");
 
-struct CmdFormatterRun : SourceExprCommand
+struct CmdFormatterRun : SourceExprCommand, MixJSON
 {
     std::vector<std::string> args;
 
@@ -87,6 +89,80 @@ struct CmdFormatterRun : SourceExprCommand
 
 static auto rFormatterRun = registerCommand2<CmdFormatterRun>({"formatter", "run"});
 
+struct CmdFormatterBuild : SourceExprCommand
+{
+    Path outLink = "result";
+
+    CmdFormatterBuild()
+    {
+        addFlag({
+            .longName = "out-link",
+            .shortName = 'o',
+            .description = "Use *path* as prefix for the symlink to the build result. It defaults to `result`.",
+            .labels = {"path"},
+            .handler = {&outLink},
+            .completer = completePath,
+        });
+
+        addFlag({
+            .longName = "no-link",
+            .description = "Do not create symlink to the build results.",
+            .handler = {&outLink, Path("")},
+        });
+    }
+
+    std::string description() override
+    {
+        return "build the current flake's formatter";
+    }
+
+    std::string doc() override
+    {
+        return
+#include "formatter-build.md"
+            ;
+    }
+
+    Category category() override
+    {
+        return catSecondary;
+    }
+
+    Strings getDefaultFlakeAttrPaths() override
+    {
+        return Strings{"formatter." + settings.thisSystem.get()};
+    }
+
+    Strings getDefaultFlakeAttrPathPrefixes() override
+    {
+        return Strings{};
+    }
+
+    void run(ref<Store> store) override
+    {
+        auto evalState = getEvalState();
+        auto evalStore = getEvalStore();
+
+        auto installable_ = parseInstallable(store, ".");
+        auto & installable = InstallableValue::require(*installable_);
+        auto unresolvedApp = installable.toApp(*evalState);
+        auto app = unresolvedApp.resolve(evalStore, store);
+
+        Installables installableContext;
+        for (auto & ctxElt : unresolvedApp.unresolved.context)
+            installableContext.push_back(make_ref<InstallableDerivedPath>(store, DerivedPath{ctxElt}));
+        auto buildables = Installable::build(evalStore, store, Realise::Outputs, installableContext);
+
+        if (outLink != "")
+            if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())
+                createOutLinks(outLink, toBuiltPaths(buildables), *store2);
+
+        logger->cout("%s", app.program);
+    };
+};
+
+static auto rFormatterBuild = registerCommand2<CmdFormatterBuild>({"formatter", "build"});
+
 struct CmdFmt : CmdFormatterRun
 {
     void run(ref<Store> store) override