fix: lazily resolve proxy environment variables #2756

Job	Run time
downstream	10m 9s
jvm (8)	3m 33s
jvm (11)	3m 21s
protocol-tests	9m 34s
all-platforms (ubuntu-latest)	5m 31s
jvm (17)	3m 40s
all-platforms (macos-latest)	13m 17s
jvm (21)	3m 37s
all-platforms (windows-latest)	8m 13s
	1h 0m 55s

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