Go: Support `GOPROXY` via the Dependabot proxy #4100

Job	Run time
Diagnostic export (ubuntu-latest, nightly-latest)	1m 8s
Diagnostic export (windows-latest, linked)	3m 38s
Diagnostic export (windows-latest, nightly-latest)	4m 40s
Diagnostic export (ubuntu-latest, linked)	1m 0s
Diagnostic export (macos-latest, nightly-latest)	1m 3s
Diagnostic export (macos-latest, linked)	1m 16s
	12m 45s

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