ci: remove obsoleted if/when-let and shadowed dynamic variable and fix some byte-compile warnings #5494

Job	Run time
test (ubuntu-latest, 27.2, false)	2m 6s
test (ubuntu-latest, 28.2, false)	2m 29s
test (ubuntu-latest, 29.4, false)	2m 55s
test (macos-latest, 28.2, false)	5m 11s
test (macos-latest, 29.4, false)	4m 2s
test (windows-latest, 27.2, false)	4m 42s
test (windows-latest, 28.2, false)	3m 13s
test (windows-latest, 29.4, false)	6m 24s
test (ubuntu-latest, snapshot, true)	1m 37s
test (macos-latest, snapshot, true)	3m 32s
test (windows-latest, snapshot, true)	9m 3s
	45m 14s

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