[ci] Use sccache in GitHub Actions

[driazati]

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Sometimes GitHub Actions takes longer than Jenkins but we can improve this bottleneck by using sccache which should eliminate most of the runtime for Windows/MacOS. We need to:

1. Configure 2 S3 buckets, one for Windows and one for MacOS (publicly readable, so jobs on main can write to it but not jobs on PRs)
2. Add credentials to access the bucket to the repo
3. Install sccache on the GitHub Actions workflows and configure it to use the relevant S3 bucket
4. Set up conda build to use sccache

cc @Mousius @areusch