ci: fix clang-13 installation
    
    For some reason Ubuntu Focal repositories now have `llvm-13` virtual
    package which can't be installed, but successfully fools our check,
    resulting in no clang/llvm being installed...
    
    ```
    $ apt show llvm-13
    Package: llvm-13
    State: not a real package (virtual)
    N: Can't select candidate version from package llvm-13 as it has no candidate
    N: Can't select versions from package 'llvm-13' as it is purely virtual
    N: No packages found
    
    $ apt install --dry-run llvm-13
    Reading package lists... Done
    Building dependency tree
    Reading state information... Done
    Package llvm-13 is not available, but is referred to by another package.
    This may mean that the package is missing, has been obsoleted, or
    is only available from another source
    
    E: Package 'llvm-13' has no installation candidate
    ```