action: the install step fails on ubuntu-latest (Ubuntu 26.04) #256

The GitHub Action (action.yml) does not run on Ubuntu 26.04 runners, which ubuntu-latest now selects.

What happens

The "Install LLVM" step fails before anything is built:

sudo: 'apt-key': command not found

apt-key has been removed from the Ubuntu 26.04 image. Even with the key installed through a keyring (/etc/apt/keyrings/llvm.gpg and deb [signed-by=...]), the next line fails: apt.llvm.org publishes no llvm-toolchain-resolute-20 branch for Ubuntu 26.04 (https://apt.llvm.org/resolute/dists/ lists the unversioned branch and 21, 22, 23 only), while the analyzer requires LLVM 20 (cmake/CheckLLVMVersion.cmake).

The repository's own workflows are pinned to ubuntu-24.04 since the fix of the CI, where llvm-toolchain-noble-20 exists; consumers of the action on ubuntu-latest hit the failure above.

Expected

The action installs its toolchain on the current ubuntu-latest:

• replace apt-key add with a keyring and signed-by, which also works on 22.04 and 24.04;
• either build the analyzer against LLVM 21 or later and let llvm-version select it, or make the action fail early with a message that names the supported runner images.

Seen in

PR #246, second push: "Test GitHub Action" and "Test Consumer Project" failed at the install step; the same jobs passed the day before on ubuntu-24.04.