Verify plugins are loadable in Windows installer builds and fix missing dependencies

[acolwell]

Thanks for submitting a pull request! Please provide enough information so that others can review your pull request. Additionally, make sure you've done all of these things:

• [x] I've followed the contributing guidelines to the best of my understanding
• [x] I've read and understood the contributing guidelines
• [x] I've formatted my code according to Natron's code style
• [x] I've searched the pull requests tracker to ensure that this PR is not a duplicate

PR Description

What type of PR is this? (Check one of the boxes below)

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] My change requires a change to the documentation
  • [ ] I have updated the documentation accordingly

What does this pull request do?

• Adds logic to the "Build Installer" GitHub action that verifies that the plugins that are built are actually loadable. This is intended to catch missing dependencies.
• Added missing libheif dependencies that were preventing the Arena and IO plugins from loading.

Have you tested your changes (if applicable)? If so, how?

Yes. I tested that the new logic properly detects that the plugins were not loadable and causes the build to fail (https://github.com/acolwell/Natron/actions/runs/7935008735). I then added the missing dependencies and verified that the build started passing again (https://github.com/acolwell/Natron/actions/runs/7935721812).

Futher details of this pull request

I'm not exactly sure when libheif took on the new dependencies, but I do know my nightly builds were passing even though the plugins had missing dependencies. This change is intended to make it easier to detect when these new dependencies sneak in and break plugin loading.

[acolwell]

Thank you for the quick review. :)