Properly unset proxy env if empty #3269

Job	Run time
Resolve environment (windows-latest, default)	1m 59s
Resolve environment (macos-latest, default)	2m 31s
Resolve environment (ubuntu-latest, linked)	33s
Resolve environment (windows-latest, nightly-latest)	4m 11s
Resolve environment (macos-latest, nightly-latest)	2m 11s
Resolve environment (windows-latest, linked)	3m 0s
Resolve environment (ubuntu-latest, nightly-latest)	34s
Resolve environment (macos-latest, linked)	2m 13s
Resolve environment (ubuntu-latest, default)	31s
	17m 43s

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