nix flake prefetch: Add --out-link option

This makes `nix flake prefetch` more useful for scripting and prevents
the result from being GC'ed prematurely.

src/nix/flake-prefetch.md

@@ -5,10 +5,14 @@ R""(
 * Download a tarball and unpack it:
 
   ```console
-  # nix flake prefetch https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.5.tar.xz
+  # nix flake prefetch https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.5.tar.xz --out-link ./result
   Downloaded 'https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.5.tar.xz?narHash=sha256-3XYHZANT6AFBV0BqegkAZHbba6oeDkIUCDwbATLMhAY='
   to '/nix/store/sl5vvk8mb4ma1sjyy03kwpvkz50hd22d-source' (hash
   'sha256-3XYHZANT6AFBV0BqegkAZHbba6oeDkIUCDwbATLMhAY=').
+
+  # cat ./result/README
+  Linux kernel
+  …
   ```
 
 * Download the `dwarffs` flake (looked up in the flake registry):

src/nix/flake.cc

@@ -18,6 +18,7 @@
 #include "markdown.hh"
 #include "users.hh"
 #include "fetch-to-store.hh"
+#include "local-fs-store.hh"
 
 #include <filesystem>
 #include <nlohmann/json.hpp>
@@ -1430,8 +1431,18 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
 
 struct CmdFlakePrefetch : FlakeCommand, MixJSON
 {
+    std::optional<std::filesystem::path> outLink;
+
     CmdFlakePrefetch()
     {
+        addFlag({
+            .longName = "out-link",
+            .shortName = 'o',
+            .description = "Create symlink named *path* to the resulting store path.",
+            .labels = {"path"},
+            .handler = {&outLink},
+            .completer = completePath
+        });
     }
 
     std::string description() override
@@ -1467,6 +1478,13 @@ struct CmdFlakePrefetch : FlakeCommand, MixJSON
                 store->printStorePath(storePath),
                 hash.to_string(HashFormat::SRI, true));
         }
+
+        if (outLink) {
+            if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())
+                createOutLinks(*outLink, {BuiltPath::Opaque{storePath}}, *store2);
+            else
+                throw Error("'--out-link' is not supported for this Nix store");
+        }
     }
 };