normalise spirv-std path #596

Job	Run time
Android (ubuntu-24.04, aarch64-linux-android)	8m 35s
Test (ubuntu-24.04)	8m 5s
Test (macOS-latest)	10m 13s
Test (windows-2022)	15m 44s
Difftest (windows-2022)	18m 34s
Difftest (macOS-latest)	7m 12s
cargo-deny	1m 15s
Lint	6m 37s
Compiletest (macOS-latest)	8m 31s
Compiletest (ubuntu-24.04)	8m 12s
Difftest (ubuntu-24.04)	9m 27s
Compiletest (windows-2022)	15m 23s
test_success	2s
	1h 57m 50s

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