Proposal Create less hermetic builds to get some more cache collisions for non-hermetic steps

If we were to use bazel strictly in a supported manner, we would attempt to create hermetic builds of the bitstreams and store them in a public bazel remote cache. Unfortunately, the EDA tools have required user-specific environment variables that may make cache hits nigh-impossible (PATH entry for tools, license server variable, etc.). I'm not sure if there's a way to pass environment variables to an action but exempt them from calculating the hash / cache key.

_Originally posted by @a-will in https://github.com/lowRISC/opentitan/issues/16863#issuecomment-1353942187_