Don’t require NIX_PATH entries to be valid paths

It’s totally valid to have entries in `NIX_PATH` that aren’t valid paths
(they can even be arbitrary urls or `channel:<channel-name>`).

Fix #5998 and #5980

tests/local.mk

@@ -63,6 +63,7 @@ nix_tests = \
   eval-store.sh \
   readfile-context.sh \
   store-ping.sh \
+  nix_path.sh \
   why-depends.sh
   # parallel.sh
 

tests/nix_path.sh

@@ -0,0 +1,11 @@
+# Regression for https://github.com/NixOS/nix/issues/5998 and https://github.com/NixOS/nix/issues/5980
+
+source common.sh
+
+export NIX_PATH=non-existent=/non-existent/but-unused-anyways:by-absolute-path=$PWD:by-relative-path=.
+
+nix-instantiate --eval -E '<by-absolute-path/simple.nix>' --restrict-eval
+nix-instantiate --eval -E '<by-relative-path/simple.nix>' --restrict-eval
+
+# Should ideally also test this, but there’s no pure way to do it, so just trust me that it works
+# nix-instantiate --eval -E '<nixpkgs>' -I nixpkgs=channel:nixos-unstable --restrict-eval