Go: Support `GOPROXY` via the Dependabot proxy #4643

Job	Run time
Go: tracing with custom build steps (macos-latest, stable-v2.16.6)	2m 13s
Go: tracing with custom build steps (ubuntu-latest, stable-v2.16.6)	1m 6s
Go: tracing with custom build steps (macos-latest, stable-v2.15.5)	2m 20s
Go: tracing with custom build steps (ubuntu-latest, stable-v2.15.5)	1m 10s
Go: tracing with custom build steps (ubuntu-latest, stable-v2.17.6)	1m 7s
Go: tracing with custom build steps (macos-latest, linked)	2m 14s
Go: tracing with custom build steps (macos-latest, stable-v2.18.4)	2m 25s
Go: tracing with custom build steps (ubuntu-latest, default)	58s
Go: tracing with custom build steps (macos-latest, default)	2m 18s
Go: tracing with custom build steps (ubuntu-latest, stable-v2.18.4)	1m 23s
Go: tracing with custom build steps (macos-latest, stable-v2.19.4)	1m 52s
Go: tracing with custom build steps (ubuntu-latest, stable-v2.19.4)	1m 21s
Go: tracing with custom build steps (macos-latest, stable-v2.17.6)	2m 35s
Go: tracing with custom build steps (ubuntu-latest, linked)	56s
Go: tracing with custom build steps (ubuntu-latest, nightly-latest)	1m 0s
Go: tracing with custom build steps (macos-latest, nightly-latest)	2m 37s
	27m 35s

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