Delete `main-build.yml` workflow

[huntharo]

Motivations

• main-build.yml has the same code as release.yml except it skips the last step of each publish action (the actual publish of the package)
• This increases code to maintain
• The BUILD-JSII label can be used to build JSII in ci.yml on PRs when needed
• The publish step can simply be skipped on non-release builds in release.yml
• The release build be triggered on PRs and pushes to main now, with a conditional that only proceeds if BUILD-RELEASE label is set on the PR

To-Do

• Remove main-build.yml file
• Add triggers to release.yml
• Add conditional to release.yml to only build on PRs if label set
• Add conditional to skip the publish steps when not a release trigger
• Remove badge from README.md