skip go tests when go is not available

[cornfeedhobo]

Description

Similar to the goenv plugin, we have to test for go's actual existence
for users with goenv and no system-installed go.

Bonus: remove the $CI check that is a relic from Travis CI days.

Motivation and Context

I stumbled into this when running tests locally.

How Has This Been Tested?

locally.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[NoahGorny]

Nice one @cornfeedhobo