ci: merge bitcoin#27314, #28954, fix multiprocess builds

[kwvg]

Additional Information

• bitcoin#27314 has been backported in this PR as bitcoin#25838 (backported in dash#6384) broke Clang depends builds.

• bitcoin#28954 has been backported to fix a problem associated with multiprocess runs (build).

• Support for multiprocess builds were enabled proper in dash#6143 but unfortunately, the configuration params for multiprocess builds were not processed by CI as the build variant was not added to matrix.sh (source). This is evident by comparing two variants with Boost::Process enablement (--with-boost-process), linux64_fuzz (source) and linux64_multiprocess (source).

  Looking at a develop (6a51ab2) build, the fuzz build has it enabled (source) while the multiprocess build doesn't (source) despite both scripts having the enablement argument.

Breaking Changes

None expected.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas (note: N/A)
• I have added or updated relevant unit/integration/functional/e2e tests (note: N/A)
• I have made corresponding changes to the documentation (note: N/A)
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[PastaPastaPasta]

utACK 27d9763

[UdjinM6]

utACK 27d9763