ci: intermediary steps use actions

[alandefreitas]

Adjust CI jobs to use actions for intermediary steps:

• Custom packages and targets included directly in the matrix only once.
• Dependencies use cmake-workflows directly.
• In PRs and local runs, when there are no cached LLVM binaries, these are downloaded directly from the MrDocs releases instead of rebuilding LLVM.
• LLVM release matrix only includes workflows for binaries missing from the server.