Fix Xcode DEVELOPER_DIR in CI #3142

Job	Run time
Check formatting	11s
Check MSRV	48s
Lint macOS x86_64	2m 38s
Lint iOS ARMv7s	2m 21s
Lint visionOS Aarch64 simulator	2m 15s
Lint GNUStep + exceptions	1m 22s
Lint header-translator	29s
Lint test-assembly	26s
Test macOS 15 Aarch64	1m 21s
Verify header translator output	1m 15s
Assembly tests	51s
Compiler UI	42s
Check framework features	0s
${{ ((matrix.test && 'Test') \|\| 'Build') }} ${{ matrix.name }}	0s
Test Compiler-RT	26s
Test GNUStep with libobjc2 v1.9	58s
Test GNUStep with libobjc2 v2.0	1m 16s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.1 on nightly	53s
Test GNUStep 32bit	1m 20s
	19m 32s

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