docs: specify that flake.lock files are JSON (#11594) #12588

Job	Run time
vm_tests	17m 58s
Check Cachix and Docker secrets present for installer tests	1s
tests (ubuntu-latest)	27m 38s
tests (macos-latest)	31m 39s
flake_regressions	24m 22s
docker_push_image	2m 2s
installer	47m 24s
installer_test (ubuntu-latest)	28s
installer_test (macos-latest)	1m 4s
	2h 32m 36s

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