Make more string values work as installables

As discussed in #7417, it would be good to make more string values work
as installables. That is to say, if an installable refers to a value,
and the value is a string, it used to not work at all, since #7484, it
works somewhat, and this PR make it work some more.

The new cases that are added for `BuiltPath` contexts:

- Fixed input- or content-addressed derivation:

  ```
  nix-repl> hello.out.outPath
  "/nix/store/jppfl2bp1zhx8sgs2mgifmsx6dv16mv2-hello-2.12"

  nix-repl> :p builtins.getContext hello.out.outPath
  { "/nix/store/c7jrxqjhdda93lhbkanqfs07x2bzazbm-hello-2.12.drv" = { outputs = [ "out" ]; }; }

  The string matches the specified single output of that derivation, so
  it should also be valid.

- Floating content-addressed derivation:

  ```
  nix-repl> (hello.overrideAttrs (_: { __contentAddressed = true; })).out.outPath
  "/1a08j26xqc0zm8agps8anxpjji410yvsx4pcgyn4bfan1ddkx2g0"

  nix-repl> :p builtins.getContext (hello.overrideAttrs (_: { __contentAddressed = true; })).out.outPath
  { "/nix/store/qc645pyf9wl37c6qvqzaqkwsm1gp48al-hello-2.12.drv" = { outputs = [ "out" ]; }; }
  ```

  The string is not a path but a placeholder, however it also matches
  the context, and because it is a CA derivation we have no better
  option. This should also be valid.

We may also want to think about richer attrset based values (also
discussed in that issue and #6507), but this change "completes" our
string-based building blocks, from which the others can be desugared
into or at least described/document/taught in terms of.

Progress towards #7417

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

src/libcmd/installable-attr-path.cc

@@ -46,7 +46,15 @@ std::pair<Value *, PosIdx> InstallableAttrPath::toValue(EvalState & state)
 
 DerivedPathsWithInfo InstallableAttrPath::toDerivedPaths()
 {
-    auto v = toValue(*state).first;
+    auto [v, pos] = toValue(*state);
+
+    if (std::optional derivedPathWithInfo = trySinglePathToDerivedPaths(
+        *v,
+        pos,
+        fmt("while evaluating the attribute '%s'", attrPath)))
+    {
+        return { *derivedPathWithInfo };
+    }
 
     Bindings & autoArgs = *cmd.getAutoArgs(*state);