Fix handling of environment variables in `RCCEnvironment` #2806

Job	Run time
analyse_changes	6s
rcc / analyse_changes	8s
rcc / check_cache	2s
tests / x86_64-pc-windows-gnu (windows-latest)	3m 3s
tests / x86_64-unknown-linux-gnu (ubuntu-latest)	45s
build_robotmk	0s
rcc / build_and_cache	0s
rcc / upload	5s
system_tests / windows	11m 8s
system_tests / linux	4m 20s
check_success	3s
	19m 40s

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