ci: fix clang-13 installation

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
```