Go: Support `GOPROXY` via the Dependabot proxy #4101

Job	Run time
Diagnostic export (macos-latest, linked)	1m 8s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 4s
Diagnostic export (windows-latest, nightly-latest)	4m 9s
Diagnostic export (windows-latest, linked)	2m 37s
Diagnostic export (ubuntu-latest, linked)	1m 0s
Diagnostic export (macos-latest, nightly-latest)	59s
	10m 57s

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