Scripts to enable local build from github actions workflow using nektos/act

[SamuelBayliss]

These script enable a containerized build locally using nektos/act. If these are widely adopted we can have many people able to replicate the CI flow exactly in their local machine ( with an environment controlled using the same mechanisms as the CI).

These scripts contain some additions to make this go fast.

• CCache usage backed by a persistent container
• Worktree checkout enabled by a bind-mount of LLVM
• Substitution of local pre-built development container, rather than fetching packages on each build.

[makslevental]

My 2cents: the solution to mismatch between CI and local is not to make local more like CI but to make CI more like local. Going down the act road leads to you inevitably trying discern the differences between that environment and GHAs when act breaks.