Recast ci modernization

[OursCodeur]

The four VS2019 jobs in the Build matrix can't start any more, the runners aren't available. Every branch I push sits at 14/18 with those four queued forever, so the matrix has stopped telling anyone whether Windows is fine. This :

• Updates six Premake download sites across the Build and Tests workflows.
• Uses beta8's gmake action name instead of removed gmake2.
• Updates FatalCompileWarnings to beta8's fatalwarnings syntax.
• Explicitly removes fatal warnings for Xcode, preserving the target workflow's prior behavior.