ci: fix detection of clang-11

Github images added clang-11 package, but somehow /usr/bin/clang-11
is missed. So, we better check that target is available rather than
package.

Change-Id: I377d9e709345f44ddcf37108c2f9db0e447cb761
Signed-off-by: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
1 file changed