Use Ubuntu for UI tests #3091

Job	Run time
Check formatting	10s
Check formatting	10s
Check MSRV	47s
Check MSRV	47s
Lint macOS 64bit	2m 31s
Lint macOS 64bit	2m 31s
Lint iOS 32bit	2m 6s
Lint iOS 32bit	2m 6s
Lint GNUStep + exceptions	1m 16s
Lint GNUStep + exceptions	1m 16s
Lint header-translator	26s
Lint header-translator	26s
Lint test-assembly	24s
Lint test-assembly	24s
Test macOS 15 Aarch64	1s
Test macOS 15 Aarch64	1s
Verify header translator output	1s
Verify header translator output	1s
Check framework features	0s
Check framework features	0s
Assembly tests	53s
Assembly tests	53s
${{ matrix.name }}	0s
${{ matrix.name }}	0s
Test Compiler-RT	23s
Test Compiler-RT	23s
Test GNUStep with libobjc2 v1.9	1m 2s
Test GNUStep with libobjc2 v1.9	1m 2s
Compiler UI	56s
Compiler UI	56s
Test GNUStep with libobjc2 v2.0	1m 6s
Test GNUStep with libobjc2 v2.0	1m 6s
Test on iOS simulator w. dinghy	0s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.1 on nightly	59s
Test GNUStep with libobjc2 v2.1 on nightly	59s
Test GNUStep 32bit	1m 0s
Test GNUStep 32bit	1m 0s
	28m 2s

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