Input: Replace 'locked' bool by isLocked() method

It's better to just check whether the input has all the attributes
needed to consider itself locked (e.g. whether a Git input has an
'rev' attribute).

Also, the 'locked' field was actually incorrect for Git inputs: it
would be set to true even for dirty worktrees. As a result, we got
away with using fetchTree() internally even though fetchTree()
requires a locked input in pure mode. In particular, this allowed
'--override-input' to work by accident.

The fix is to pass a set of "overrides" to call-flake.nix for all the
unlocked inputs (i.e. the top-level flake and any --override-inputs).

src/nix-env/nix-env.cc

@@ -143,7 +143,7 @@ static void getAllExprs(EvalState & state,
             }
             /* Load the expression on demand. */
             auto vArg = state.allocValue();
-            vArg->mkString(path2.path.abs());
+            vArg->mkPath(path2);
             if (seen.size() == maxAttrs)
                 throw Error("too many Nix expressions in directory '%1%'", path);
             attrs.alloc(attrName).mkApp(&state.getBuiltin("import"), vArg);