fix: use 'go env GOROOT' when GOROOT env is missing

[icholy]

Description

This change removes the need for explicitly setting the GOROOT environment variable. Recent versions of Go will output the default env values when using go env NAME.

Checklist

• [x] I have updated the PR title to match CDKTF's style guide
• [x] I have run the linter on my code locally
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation if applicable
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works if applicable
• [x] New and existing unit tests pass locally with my changes

[github-actions[bot]]

I'm going to lock this pull request because it has been closed for 30 days. This helps our maintainers find and focus on the active issues. If you've found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.