fix(CI): On musl/alpine builds, pin llvm to llvm20, add llvm bin directory to path (#81519)
Alpine Edge recently removed `llvm-config` from the PATH:
- 3.22:
https://pkgs.alpinelinux.org/contents?file=llvm-config&path=&name=&branch=v3.22&repo=&arch=x86_64
- Edge:
https://pkgs.alpinelinux.org/contents?file=llvm-config&path=&name=&branch=edge&repo=&arch=x86_64
CI job: https://github.com/vercel/next.js/actions/runs/16208743604
Also, fix a bunch of escaping problems with how we invoke bash inside of
docker...
Co-authored-by: Will Binns-Smith <wbinnssmith@gmail.com>