Add flag `set-env-var` to `MixEnvironment`, and rename old ones #12432

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
vm_tests	17m 18s
tests (ubuntu-latest)	27m 11s
tests (macos-latest)	51m 33s
flake_regressions	18m 44s
docker_push_image	0s
installer	0s
installer_test	0s
	1h 54m 47s

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