This fixes the bug introduced by 8686b98612 which broke bundlerEnv
exprs when gemdir was a string (thus making gemset a string by
`gemset = gemdir + "/gemset.nix"`) which made it being treated as a
set.
This allows patching the gemset output by bundix from a default.nix
file, making it easier to perform updates since the bundix update no
longer has to be manually updated.