Add Weekly CI

[hmelder]

This CI uses the latest LLVM Snapshot from apt.llvm.org and runs weekly on Monday morning. Additionally, we build from the libdispatch main branch.

[hmelder]

With clang-22:

/home/runner/work/libobjc2/libobjc2/Test/FastPathAlloc.m:99:9: error: incompatible pointer types returning 'ShouldInit *' from a function with result type 'ShouldInitSubclassed *' [-Wincompatible-pointer-types]
   99 |         return [ShouldInit alloc];

[hmelder]

Addresses #269