CONTRIBUTING.md: add note for fixture update (#1350) #2578

Job	Run time
Lints	4m 24s
build-aarch64-linux / Build aarch64 Linux (static)	10m 20s
build-aarch64-darwin / Build aarch64 Darwin	8m 4s
build-x86_64-darwin / Build x86_64 Darwin	11m 52s
build-x86_64-linux / Build x86_64 Linux (static)	6m 56s
Run x86_64 Linux (--determinate)	2m 9s
Run x86_64 Linux release check matrix	0s
Run x86_64 Linux (No init, --determinate)	1m 37s
Run x86_64 Linux	1m 44s
Run x86_64 Linux (No init)	1m 32s
Run x86_64 Linux release checks	0s
Run aarch64 Darwin (--determinate)	2m 5s
Run aarch64 Darwin	2m 10s
Run aarch64 Linux (--determinate)	1m 40s
Run aarch64 Linux	1m 19s
Run x86_64 Darwin (--determinate)	5m 19s
Run x86_64 Darwin	4m 3s
	1h 5m 14s

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