The previous implementation would allow variables to sneak into the
file names. This commit makes sure the resulting target file path
exactly matches the expected path.
Using this function it is possible to make `home.file` create a
symlink to a path outside the Nix store. For example, a Home Manager
configuration containing
home.file."foo".source = config.lib.file.mkOutOfStoreSymlink ./bar;
would upon activation create a symlink `~/foo` that points to the
absolute path of the `bar` file relative the configuration file.
PR #1211
