Revert "lib/trivial: fix 'error: cannot decode virtual path '/nix/store/virtual0000000000000000000000005-source''"

This reverts commit b67ee6e861.

https://github.com/NixOS/nixpkgs/issues/202244

error: a string that refers to a store path cannot be appended to a path, at /etc/nixos/nix/nixos-unstable/lib/sources.nix:193:30

appears to happen when there's a nixpkgs git submodule

> So one of the things that is different for a git submodule is that the .git folder isn't a folder, it's a textfile that contains (in my case) this:

> $ cat nix/nixos-unstable/.git
> gitdir: ../../.git/modules/nixpkgs

lib/trivial.nix

@@ -213,8 +213,8 @@ rec {
     # Default value to return if revision can not be determined
     default:
     let
-      revisionFile = ./.. + "/.git-revision";
-      gitRepo      = ./.. + "/.git";
+      revisionFile = "${toString ./..}/.git-revision";
+      gitRepo      = "${toString ./..}/.git";
     in if lib.pathIsGitRepo gitRepo
        then lib.commitIdFromGitRepo gitRepo
        else if lib.pathExists revisionFile then lib.fileContents revisionFile