[ci][docker] Conditionally link sccache to clang (#11316)

This was causing errors with #11314 since it was making it appear as if `clang` was available when it was only the sccache wrapper.

Co-authored-by: driazati <[email protected]>

Author: driazati

docker/install/ubuntu_install_sccache.sh

@@ -26,8 +26,14 @@ cargo install sccache
 mkdir /opt/sccache
 ln "$(which sccache)" /opt/sccache/cc
 ln "$(which sccache)" /opt/sccache/c++
-ln "$(which sccache)" /opt/sccache/clang
-ln "$(which sccache)" /opt/sccache/clang++
+
+# Only add clang if it's on the PATH
+if command -v clang &> /dev/null
+then
+    ln "$(which sccache)" /opt/sccache/clang
+    ln "$(which sccache)" /opt/sccache/clang++
+fi
+
 
 # make rust usable by all users after install during container build
 chmod -R a+rw /opt/rust