Add flag `set-env-var` to `MixEnvironment`, and rename old ones #12910

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
vm_tests	17m 19s
tests (ubuntu-latest)	25m 11s
tests (macos-latest)	27m 47s
flake_regressions	11m 2s
docker_push_image	0s
installer	0s
installer_test	0s
	1h 21m 20s

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