ci: build multiprocess on most jobs
This builds most of the CI jobs with multiprocess enabled.
Two jobs run the functional test suite, one with depends and one with the regular build.
The Windows job does not use multiprocess since that's not supported yet.
The previous version of this PR changed the depends system to build libmultiprocess by default as one of the final steps for #31098 - but based discussion here and on IRC it was decided it's too early to ship it.
Based on #31741