EXTRA_NIX_ARGS are not universal (e.g.: `--rebuild`)

[srid]

Consider (after #439),

om ci run github:srid/haskell-template -- --refresh --override-input flake-parts github:hercules-ci/flake-parts --rebuild
...

❄️  nix eval --refresh --override-input flake-parts github:hercules-ci/flake-parts --rebuild --json 'github:srid/haskell-template#om' --quiet --quiet️
Error: Nix command error: Command error: Process exited unsuccessfully. exit_code=Some(1) stderr=error: unrecognised flag '--rebuild'
Try 'nix --help' for more information.

Here, we pass --rebuild, but that doesn't make sense in all Nix commands, such as nix eval. But omnix unconditionally passes them to all anyway.

[srid]

#441 is an attempt to address this, but I'm not really happy with it, so leaving this open.