CONTRIBUTING.md: add note for fixture update #2576

Job	Run time
Lints	2m 46s
build-aarch64-darwin / Build aarch64 Darwin	8m 3s
build-x86_64-darwin / Build x86_64 Darwin	12m 56s
build-aarch64-linux / Build aarch64 Linux (static)	10m 13s
build-x86_64-linux / Build x86_64 Linux (static)	7m 11s
Run x86_64 Linux (No init, --determinate)	1m 0s
Run x86_64 Linux (No init)	1m 6s
Run x86_64 Linux (--determinate)	1m 19s
Run x86_64 Linux release check matrix	0s
Run x86_64 Linux	1m 0s
Run x86_64 Linux release checks	0s
Run aarch64 Darwin (--determinate)	2m 4s
Run aarch64 Darwin	2m 8s
Run aarch64 Linux (--determinate)	1m 33s
Run aarch64 Linux	1m 14s
Run x86_64 Darwin (--determinate)	5m 29s
Run x86_64 Darwin	4m 19s
	1h 2m 21s

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