CI: Use a Ubuntu 20.04 container to keep Ubuntu 20.04 support #1961

Job	Run time
build-spidermonkey-win	11s
sdist	31s
build-spidermonkey-unix (ubuntu-22.04, 3.10)	3m 24s
build-spidermonkey-unix (ubuntu-22.04-arm, 3.10)	21s
build-spidermonkey-unix (macos-13, 3.10)	1s
build-spidermonkey-unix (macos-14, 3.10)	1s
build-and-test	1s
publish-nightly	1s
publish	1s
publish-archive	1s
	4m 33s

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