Variable cache compression level

[nyanrus]

Thank you for good project!

It will be good if I can change the compression level with Environment Variable.
Currently, the cache level seems constant 3, default level.
https://github.com/mozilla/sccache/blob/main/src%2Fcache%2Fcache.rs#L293-L293

I am using sccache in GitHub Actions to make cache for building a firefox fork.
And the GitHub Actions Cache is only 10GB.
So the higher compression level is preferred because of lacking cache storage size.

Thank you for reading this issue!

[sylvestre]

Don't hesitate to submit a PR for this :)
It should be easy to implement

[nyanrus]

Ok, I will try in this week.