WIP Fix for LOCALLY parallelism

[vladaionescu]

LOCALLY targets should not execute in parallel. This PR fixes that by introducing a global lock.

This is WIP - it seems that this is introducing a deadlock for now. Needs more work.

[idelvall]

LOCALLY targets should not execute in parallel

Why is that? :)

[vladaionescu]

It's a design principle - that things should be as repeatable as possible. We figured that kinda like how you rarely run make with the -j argument because it's hard to get parallelism right when the environment is shared, we should avoid parallelizing in LOCALLY cases in Earthly too.

It's probably not an important guardrail now that there is also WAIT, which allows the user to explicitly control parallelism where necessary. Plus nobody has asked for this feature, so we could also abandon it. CC @alexcb