ci: use GOPROXY fallbacks

[burgerdev]

Context

Golang builds use a module proxy to fetch dependencies. The default configuration is GOPROXY=https://proxy.golang.org,direct, which means "always use proxy.golang.org, and fall back to direct if the result is a 404". If the proxy fails differently (network disconnect, 502, ...), fetching fails.

Proposed change(s)

• Use fallbacks on all proxy errors (this is done by using | instead of ,).
• Add goproxy.io as additional fallback.

Related issue

• Fixes edgelesssys/issues#1004

Checklist

• Add labels (e.g., for changelog category)
• Is PR title adequate for changelog?
• Link to Milestone

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[burgerdev]

Unit test was successful.