fixes CI workflow configuration

[ju1ius]

The latest runs were broken for some reason so I fixed it.
Here's a summary of what this PR does:

• upgrades LLVM to v14
• migrates from the unmaintained action-rs/* actions to dtolnay/rust-toolchain, using Swatinem/rust-cache as a cache layer.
• adds a cache layer for LLVM build
• adds a weekly cron schedule for all workflows
• fixes an issue in the docblocks

[ptondereau]

One thing I like to do is to add cron every week for building + doc. That way, we can anticipate that kind of error

[ptondereau]

Thank you!