add NIX_MANIFESTS_DIR to tests

Dependencies

• [2] PVBTVPKJ nix needs env vars as absolute paths
• [*] G2ZB6464 first test, not yet in buildprocess

Change contents

• edit in tests/Makefile.am at line 9
  [2.91]

  [2.91]
  + NIX_MANIFESTS_DIR="$(abs_builddir)/nix/var/nix/manifests" \