Switch from actions/setup-go@v2-beta to actions/setup-go@v2 to fix the GitHub Actions set-env failure (#4302)
Switch from actions/setup-go@v2-beta to actions/setup-go@v2 to fix the GitHub Actions set-env failure (#4302)

This PR switches from actions/setup-go@v2-beta to actions/setup-go@v2 to fix the
GitHub Actions `set-env` failure. Without this PR the following error shown up
(see https://github.com/coredns/coredns/runs/1423802307):
```
Error: Unable to process command '::set-env name=GOROOT::/opt/hostedtoolcache/go/1.15.5/x64' successfully.
7
Error: The `set-env` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
```

Signed-off-by: Yong Tang <765086fe2e0c1f980161f127fec596800f327f62@outlook.com>
yongtang/coredns,miekg/coredns,chrisohaver/coredns,coredns/coredns,miekg/coredns