[target-spec-miette] switch fixtures to snapbox #1441

Job	Run time
Cross-compile to aarch64	1m 36s
Build and test (all features) (ubuntu-latest, 1.78)	43s
Build documentation (ubuntu-latest)	52s
Build and test core crates (ubuntu-latest, 1.78)	1m 49s
Build and test (all features) (ubuntu-latest, stable)	28s
Build and test core crates (ubuntu-latest, stable)	1m 30s
Lint	54s
Build and test (all features) (macos-14, 1.78)	47s
cargo-compare extended tests (ubuntu-latest)	2m 36s
Build and test core crates (macos-14, 1.78)	1m 27s
Build and test (all features) (macos-14, stable)	30s
cargo-compare extended tests (macos-14)	2m 42s
Build and test core crates (macos-14, stable)	1m 19s
Build and test (all features) (windows-latest, 1.78)	2m 22s
Build and test core crates (windows-latest, 1.78)	4m 26s
Build and test (all features) (windows-latest, stable)	3m 3s
Build and test core crates (windows-latest, stable)	3m 14s
	30m 18s

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