Make eval-release.nix less picky

It now only checks that each derivation's drvPath attribute evaluates.
Previously it checked everything (via toXML) but that causes
derivations with dubious passthru attribute to fail (see commit
70fee5da112b8a0093d59ac16cdead4d0a72f357).
Drop --xml, it's no longer needed
polkit, pulseaudio: Drop some Linux-specific dependencies