Refactor browser and metal examples to use OnceCell #2174

Job	Run time
Check MSRV	1m 53s
Check formatting	16s
Lint macOS 64bit	4m 3s
Lint iOS 32bit	1m 16s
Lint GNUStep + exceptions	1m 42s
Lint header-translator	32s
Lint test-assembly	31s
Test macOS 12	0s
Assembly tests	0s
Verify header translator output	0s
Check icrate features	0s
Compiler UI	0s
${{ matrix.name }}	0s
Test Compiler-RT	0s
${{ matrix.name }}	0s
Test on iOS simulator w. dinghy	0s
	10m 13s

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