[FR]: Allow to disable mutex while keeping global yarn cache

### What is the current behavior?

`use_global_yarn_cache = true` by default pass `--mutex network` and there's no in-between, I cannot have the use of a global cache with no mutex.

### Describe the feature

I run bazel builds making use of `rules_nodejs` in Nix, which provides an immutable offline Yarn cache.

I would like to disable the mutex while letting the "yarn install" do the trivial thing of examining that cache without worrying about thread safety, which should give a boost to my builds.

Unfortunately, I cannot find an obvious way to do this.