GitHub Actions workflows modified (remove changes as possible)

[vlevigneron]

• In order to get checks completed for zonemaster/zonemaster-engine#896 PR, worflows had been modified to only work on 'develop' branch instead 'develop' + 'latest'.
• Restore 'develop' + 'latest' checks as soon as possible.

[ghost]

The same problem occurred again recently. So 'latest' has been disabled again with #1062. What is the point in testing Zonemaster::Engine develop branch against Zonemaster::LDNS master branch?

The change has been introduced in 6b81954b from PR #890. This PR contains a discussion on testing against Zonemaster::LDNS from CPAN (see https://github.com/zonemaster/zonemaster-engine/pull/890#issuecomment-806567263 and following comments).

Since this change, this check against "latest" has been removed twice: https://github.com/zonemaster/zonemaster-engine/commit/88075c0c193fd9175727c7d1a558230d85869937 (from #896) and https://github.com/zonemaster/zonemaster-engine/commit/569dc1a7a0decda8a9507c3025449eb71b77a593 (from #1062). The first removal occurred during the same release cycle as the feature insertion.

It looks like we need to question the need for such test. Or think about the way we change things between Zonemaster::LDNS and Zonemaster::Engine.

[matsduf]

The same problem occurred again recently. So 'latest' has been disabled again with #1062. What is the point in testing Zonemaster::Engine develop branch against Zonemaster::LDNS master branch?

I am not sure, but it could be that if it fails against develop branch and not against master branch then you might find the cause.

In the Travis model develop branch is tested against develop branch (and master branch against master branch).

I suggest that we permanently keep "stable" commented, and only uncomment in case of need.