release: switch the main branch's extension target to golang.go-nightly

[hyangah]

Currently the main branch is configured to build golang.go extension.
The reasoning behind it was to help new extension developers start more easily (often
the new extension developers encountered the extension conflicts caused by their
existing golang.go installation during testing) and to test that new features
don't break the stable versions.

However, now we disable all other extensions explicitly during testing (so we avoid conflicts),
better documentation can help the extension developers to start, and we can configure the
CI or a separate CI workflow to test the stable version.

By configuring the main branch to build the nightly, we can help developers to test the nightly
version more thoroughly and easily.

[gopherbot]

Change https://golang.org/cl/283640 mentions this issue: package.json: switch the main branch in preview mode

[gopherbot]

Change https://golang.org/cl/283642 mentions this issue: .github/workflows: add preview mode check

[hyangah]

Obsolete.

go-nightly will retire in favor of the new prerelease version of the extension.
Then, this doesn't make sense.