Properly unset proxy env if empty #6412

Job	Run time
Packaging: Config file (ubuntu-latest, linked)	1m 39s
Packaging: Config file (windows-latest, linked)	5m 39s
Packaging: Config file (windows-latest, default)	5m 36s
Packaging: Config file (ubuntu-latest, default)	57s
Packaging: Config file (windows-latest, nightly-latest)	3m 15s
Packaging: Config file (ubuntu-latest, nightly-latest)	1m 41s
Packaging: Config file (macos-latest, linked)	1m 23s
Packaging: Config file (macos-latest, nightly-latest)	1m 28s
Packaging: Config file (macos-latest, default)	1m 30s
	23m 8s

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