Use Ubuntu for UI tests #3090

Job	Run time
Lint macOS 64bit	2m 23s
Lint iOS 32bit	2m 5s
Check MSRV	57s
Check formatting	12s
Lint GNUStep + exceptions	1m 23s
Lint header-translator	32s
Lint test-assembly	40s
Test macOS 15 Aarch64	1s
Check framework features	1s
Assembly tests	3m 26s
Test macOS 12 x86_64	1s
Verify header translator output	1s
Test Compiler-RT	1m 10s
Test macOS 15 AArch64	1s
Test GNUStep with libobjc2 v1.9	2m 8s
Test on iOS simulator w. dinghy	10m 11s
Test macOS 14 Aarch64 nightly	1s
Test GNUStep with libobjc2 v2.0	2m 10s
Compiler UI	49s
Build macOS 32bit	1s
Test GNUStep with libobjc2 v2.1 on nightly	1m 54s
Build iOS simulator Aarch64	1s
Test GNUStep 32bit	1m 28s
Build iOS Aarch64	1s
Build iOS ARMv7s	1s
Build iOS x86	1s
Test macOS 14 x86_64 with 10.12 SDK	1s
	31m 40s

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