Updating CI workflow on Windows to use Natron pacman repo for deps.

[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)

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] 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?

It changes the CI workflow to use the Natron pacman repo like the "Build Installer" workflow does. This allows both workflows to benefit from fixes made in the Natron pacman repo.

• Adding a step to the Windows workflow that downloads and sets up the Natron pacman repo. This is copied from the "Build Installer" workflow.
• Removed dependencies from the "Install Windows system packages" that are installed when the mingw-w64-x86_64-natron-build-deps-qt5 package is installed.

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

Yes. I've verified that the "Tests" action in the CI workflow builds again on Windows. (https://github.com/acolwell/Natron/actions/runs/7350002125)

Futher details of this pull request

This change is needed so that #935 also fixes the CI "Tests" workflow for Windows. All workflows should now pass.

[acolwell]

Thanks for the quick review @rodlie