CI: Use a Ubuntu 20.04 container to keep Ubuntu 20.04 support #1963

Job	Run time
build-spidermonkey-win	17s
build-spidermonkey-unix (ubuntu-22.04, 3.10)	2m 4s
build-spidermonkey-unix (ubuntu-22.04-arm, 3.10)	17s
build-spidermonkey-unix (macos-13, 3.10)	9s
build-spidermonkey-unix (macos-14, 3.10)	20s
sdist	29s
publish	0s
publish-nightly	0s
build-and-test	0s
publish-archive	0s
build-spidermonkey-unix (ubuntu-22.04, 3.10)	1m 51s
sdist	29s
build-spidermonkey-unix (macos-13, 3.10)	9s
build-spidermonkey-unix (macos-14, 3.10)	20s
build-spidermonkey-unix (ubuntu-22.04-arm, 3.10)	17s
build-spidermonkey-win	17s
build-and-test	0s
publish	0s
publish-nightly	0s
publish-archive	0s
	6m 59s

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