lib flake: Fix version

Manually tested with

nix-repl> :lf path:lib
nix-repl> lib.version

lib/tests/test-with-nix.nix

@@ -49,6 +49,15 @@ pkgs.runCommand "nixpkgs-lib-tests-nix-${nix.version}" {
 
   nix-store --init
 
+  # nixpkgs/lib/.version is a copy of nixpkgs/.version, so that the lib
+  # subtree is valid in isolation (except for the tests, but that's ok)
+  cp ${../../.version} nixpkgs-.version
+  cp ${../.version} nixpkgs-lib-.version
+  diff -U3 nixpkgs-.version nixpkgs-lib-.version || {
+    echo '<nixpkgs/.version> and <nixpkgs/lib/.version> must match.'
+    exit 1
+  }
+
   cp -r ${../.} lib
   echo "Running lib/tests/modules.sh"
   bash lib/tests/modules.sh