Add flag `set-env-var` to `MixEnvironment`, and rename old ones #12430

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
vm_tests	17m 14s
tests (ubuntu-latest)	26m 27s
tests (macos-latest)	37m 48s
flake_regressions	31m 26s
docker_push_image	0s
installer	0s
installer_test	0s
	1h 52m 56s

Provide feedback