Windows: Download GTK instead of building

[danyeaw]

This PR updates the CI/CD pipeline and dev environment instructions to directly download a pre-built version of GTK, rather than building it from source ourselves. The release of Gvsbuild today is the first one that distributed GTK as a release asset.

Also Step Security's Harden Runners only work with Ubuntu VMs, so I removed them for the Windows, macOS, and Fedora workflows.

PR Checklist

Please check if your PR fulfills the following requirements:

• I have read, and I am following the Contributor guide
• I have read, and I understand the GNOME Code of Conduct

PR Type

What kind of change does this PR introduce?

• Bug fix
• Feature
• Chore (refactoring, formatting, local variables, other cleanup)
• Documentation content changes

What is the current behavior?

We have to rebuild GTK ourselves for Gaphor on Windows

Issue Number: N/A

What is the new behavior?

We download GTK and use it

Does this PR introduce a breaking change?

• Yes
• No

Other information

[amolenaar]

Looks good. This is a nice simplification on our end.

Does the Gvsbuild zip file contain everything that's part of that repo?

[danyeaw]

Hi @amolenaar it isn't everything, it is the core GTK libraries including libadwaita and GtkSourceView with gi enabled.

[amolenaar]

This looks good. A nice simplification for people who want to develop on Gaphor on Windows.