Add flag `set-env-var` to `MixEnvironment`, and rename old ones #12911

Job	Run time
vm_tests	17m 1s
Check Cachix and Docker secrets present for installer tests	1s
tests (ubuntu-latest)	25m 4s
tests (macos-latest)	32m 30s
flake_regressions	17m 37s
docker_push_image	0s
installer	0s
installer_test	0s
	1h 32m 13s

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