Use Ubuntu for UI tests #3088

Job	Run time
Check formatting	14s
Lint macOS 64bit	2m 38s
Check MSRV	50s
Lint iOS 32bit	2m 10s
Lint GNUStep + exceptions	1m 35s
Lint header-translator	26s
Lint test-assembly	29s
Test macOS 12	3m 10s
Assembly tests	3m 27s
Check framework features	10m 30s
Verify header translator output	54s
Test macOS 13	5m 10s
Test Compiler-RT	32s
Test macOS 14	2m 28s
Test on iOS simulator w. dinghy	16m 53s
Build macOS AArch64	1m 54s
Test GNUStep with libobjc2 v1.9	2m 19s
Test macOS old SDK	34s
Test GNUStep with libobjc2 v2.0	2m 16s
Compiler UI	23s
Test macOS nightly	2m 55s
Test GNUStep with libobjc2 v2.1 on nightly	1m 32s
Build macOS 32bit	5m 6s
Test GNUStep 32bit	1m 32s
Build iOS simulator ARM64	1m 49s
Build iOS ARM64	1m 54s
Build iOS ARMv7s	3m 19s
Build iOS 32bit x86	2m 58s
	1h 19m 57s

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