Update JIT builds to use LLVM 20 #136895

It's that time again! Time to bump our LLVM dep to 20.1.8 since the last planned release went out a bit ago (and the installers have caught up).

Linked PRs

• GH-136895: Update JIT builds to use LLVM 20 #140329
• GH-136895: Fixes for pulling LLVM as a release artifact #141002
• [3.14] GH-136895: Update JIT builds to use LLVM 21 (GH-140329) #146419
• [3.14] GH-136895: Update JIT builds to use LLVM 21 (GH-140329) #146420