nixpkgs-basic-release-checks: set NIX_STORE_DIR

With nix 2.4 the fact that we set NIX_STATE_DIR, but kept the original
store became a hard error. We work around it here by forcing the use of
a fresh, sandbox-local, store.

See regnat's wonderful analysis for more information:
https://github.com/NixOS/nix/issues/5495#issuecomment-963949707

pkgs/top-level/nixpkgs-basic-release-checks.nix

···
 pkgs.runCommand "nixpkgs-release-checks" { src = nixpkgs; buildInputs = [nix]; } ''
     set -o pipefail
 
-    export NIX_STATE_DIR=$TMPDIR
+    export NIX_STORE_DIR=$TMPDIR/store
+    export NIX_STATE_DIR=$TMPDIR/state
     export NIX_PATH=nixpkgs=$TMPDIR/barf.nix
     opts=(--option build-users-group "")
     nix-store --init