builtins.getFlake: also support path argument

This is a papercut that has been bugging me for a while. `:lf .` is fine
but doesn't show you what variables it adds to the context. And
`builtins.getFlake (toString ./.)` works but gets tedious after a while.

When inspecting a flake in the repl, the natural thing to do is to call:

    x = builtins.getFlake ./.

Before this change, Nix would fail with:

    error: expected a string but found a path: /path/to/flake

After this change, Nix automatically coerces the path to a string.

src/libflake/flake/flake.cc

@@ -823,7 +823,11 @@ void initLib(const Settings & settings)
 {
     auto prim_getFlake = [&settings](EvalState & state, const PosIdx pos, Value * * args, Value & v)
     {
-        std::string flakeRefS(state.forceStringNoCtx(*args[0], pos, "while evaluating the argument passed to builtins.getFlake"));
+        NixStringContext context; // no context
+        auto flakeRefS = state.coerceToString(pos, *args[0], context,
+                                        "while evaluating the argument passed to builtins.getFlake",
+                                        false, false, true).toOwned();
+
         auto flakeRef = parseFlakeRef(state.fetchSettings, flakeRefS, {}, true);
         if (state.settings.pureEval && !flakeRef.input.isLocked())
             throw Error("cannot call 'getFlake' on unlocked flake reference '%s', at %s (use --impure to override)", flakeRefS, state.positions[pos]);
@@ -849,6 +853,12 @@ void initLib(const Settings & settings)
           (builtins.getFlake "nix/55bc52401966fbffa525c574c14f67b00bc4fb3a").packages.x86_64-linux.nix
           ```
 
+          The function can also be used to load flakes from the file system. For example:
+
+          ```nix
+          (builtins.getFlake ./.).packages.x86_64-linux.nix
+          ```
+
           Unless impure evaluation is allowed (`--impure`), the flake reference
           must be "locked", e.g. contain a Git revision or content hash. An
           example of an unlocked usage is:

tests/functional/flakes/shebang.sh

@@ -69,7 +69,7 @@ EOF
 cat >> "$scriptDir/shebang-inline-expr.sh" <<"EOF"
 #! nix --offline shell
 #! nix --impure --expr ``
-#! nix let flake = (builtins.getFlake (toString ../flake1)).packages;
+#! nix let flake = (builtins.getFlake ../flake1).packages;
 #! nix     fooScript = flake.${builtins.currentSystem}.fooScript;
 #! nix     /* just a comment !@#$%^&*()__+ # */
 #! nix  in fooScript

tests/functional/suggestions.sh

@@ -37,8 +37,8 @@ NIX_BUILD_STDERR_WITH_NO_CLOSE_SUGGESTION=$(! nix build .\#bar 2>&1 1>/dev/null)
 [[ ! "$NIX_BUILD_STDERR_WITH_NO_CLOSE_SUGGESTION" =~ "Did you mean" ]] || \
     fail "The nix build stderr shouldn’t suggest anything if there’s nothing relevant to suggest"
 
-NIX_EVAL_STDERR_WITH_SUGGESTIONS=$(! nix build --impure --expr '(builtins.getFlake (builtins.toPath ./.)).packages.'"$system"'.fob' 2>&1 1>/dev/null)
-[[ "$NIX_EVAL_STDERR_WITH_SUGGESTIONS" =~ "Did you mean one of fo1, fo2, foo or fooo?" ]] || \
+NIX_EVAL_STDERR_WITH_SUGGESTIONS=$(! nix build --impure --expr '(builtins.getFlake ./.).packages.'"$system"'.fob' 2>&1 1>/dev/null)
+[[ "$NIX_EVAL_STDERR_WITH_SUGGESTIONS" =~ "Did you mean one of fo1, fo2, foo or fooo?" ]] ||
     fail "The evaluator should suggest the three closest possiblities"
 
 NIX_EVAL_STDERR_WITH_SUGGESTIONS=$(! nix build --impure --expr '({ foo }: foo) { foo = 1; fob = 2; }' 2>&1 1>/dev/null)