Skip installing 'clang' if 'clang' binary already exists (#32242)

Simply installing 'clang' installs the default version of Clang for
Linux. For instance, the command: 'apt install clang' installs
'clang-14' in Ubuntu 22.04.

It might be possible that a more recent version of clang is
already installed in the system. For instance, package 'clang-17'.

In the case a 'clang' binary is already installed in the system, skip
the installation of 'clang'.

python/servo/platform/linux.py

@@ -164,6 +164,11 @@ def install_non_gstreamer_dependencies(self, force: bool) -> bool:
             command = ['apt-get', 'install', "-m"]
             pkgs = APT_PKGS
 
+            # Skip 'clang' if 'clang' binary already exists.
+            result = subprocess.run(['which', 'clang'], capture_output=True)
+            if result and result.returncode == 0:
+                pkgs.remove('clang')
+
             # Try to filter out unknown packages from the list. This is important for Debian
             # as it does not ship all of the packages we want.
             installable = subprocess.check_output(['apt-cache', '--generate', 'pkgnames'])