ci: split out `multiprocess` linting to a separate step on Actions #337

Job	Run time
Build slim container / Build container	23s
Build container / Build container	22s
x86_64-pc-linux-gnu_multiprocess / Build depends	57s
x86_64-pc-linux-gnu / Build depends	54s
arm-linux-gnueabihf / Build depends	1m 2s
x86_64-pc-linux-gnu_nowallet / Build depends	1m 2s
x86_64-apple-darwin / Build depends	1m 0s
x86_64-w64-mingw32 / Build depends	53s
win64-build / Build source	10m 12s
linux64_sqlite-build / Build source	6m 11s
linux64_fuzz-build / Build source	17m 20s
linux64_ubsan-build / Build source	6m 57s
linux64-build / Build source	5m 50s
linux64_tsan-build / Build source	11m 14s
linux64_multiprocess-build / Build source	36m 19s
mac-build / Build source	3m 28s
linux64_nowallet-build / Build source	6m 28s
arm-linux-build / Build source	3m 46s
linux64-test / Test source	34m 11s
linux64_sqlite-test / Test source	29m 37s
linux64_nowallet-test / Test source	13m 17s
linux64_ubsan-test / Test source	33m 6s
linux64_tsan-test / Test source	52m 55s
linux64_multiprocess-test / Test source	52m 57s
	5h 30m 21s

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