Add ability to customize compression level.

[dpc]

As transfers times are often the bottleneck, would be nice if it was possible to pass custom arguments to zstd compression (like -19 or even -U -30) and trade compute for storage space.

[ipetkov]

I'd be weary of adding too many customization options especially if we decide to not use zstd at some point.

FWIW it is possible to disable the default behavior by setting doInstallCargoArtifacts = false; and then using your own hook to do the compression.

It's not ergonomic, but I'd like to at least explore alternatives for optimizing the build artifacts #74 before we add more configurations here