avm2: report more correct Capabilities.os #24880

Job	Run time
Paths filter	9s
Check dependencies	1m 50s
Test Rust ${{ matrix.rust_version }} / ${{ matrix.os }}	0s
Test Rust stable / ubuntu-24.04	4m 23s
Lints with Rust stable	4m 34s
Test Rust stable / windows-latest	10m 37s
Lints with Rust beta	4m 22s
Lints with Rust nightly	4m 24s
Test Rust stable / macos-14	3m 57s
Test Rust nightly / ubuntu-24.04	8m 40s
Test Rust beta / ubuntu-24.04	4m 22s
	47m 18s

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