lib/path/tests: Fix test setup on darwin

These statements are taken from the `lib/test/release.nix` tests,
which previously also worked on darwin.

Unblocks https://github.com/NixOS/nix/pull/8569 when backported

lib/path/tests/default.nix

@@ -18,7 +18,14 @@ pkgs.runCommand "lib-path-tests" {
   ];
 } ''
   # Needed to make Nix evaluation work
-  export NIX_STATE_DIR=$(mktemp -d)
+  export TEST_ROOT=$(pwd)/test-tmp
+  export NIX_BUILD_HOOK=
+  export NIX_CONF_DIR=$TEST_ROOT/etc
+  export NIX_LOCALSTATE_DIR=$TEST_ROOT/var
+  export NIX_LOG_DIR=$TEST_ROOT/var/log/nix
+  export NIX_STATE_DIR=$TEST_ROOT/var/nix
+  export NIX_STORE_DIR=$TEST_ROOT/store
+  export PAGER=cat
 
   cp -r ${libpath} lib
   export TEST_LIB=$PWD/lib