CI: Downgrade `libloading` to `0.8.8` in MSRV test #3197

Job	Run time
Check ash, ash-window and ash-rewrite MSRV (1.69.0)	26s
Check	47s
Generated	48s
Test Suite	1m 30s
Clippy (x86_64-pc-windows-msvc)	1m 0s
Rustfmt	10s
Build-test docs	46s
Clippy (x86_64-unknown-linux-gnu)	1m 14s
Clippy (aarch64-apple-ios)	24s
Clippy (x86_64-apple-darwin)	58s
	8m 3s

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