tools: improve detection of "ci-fairy" command in "tools/check-gitlab-ci.sh"

The previous implementation would print

    ./tools/check-gitlab-ci.sh: line 22: ci-fairy: command not found

also, it's not nice to execute the program, just to find out whether
we have it.

tools/check-gitlab-ci.sh

@@ -23,7 +23,7 @@ if ! [ -f ./.gitlab-ci.yml ] ; then
     exit 0
 fi
 
-if ! ci-fairy --help 2>&1 >/dev/null ; then
+if ! command -v ci-fairy &> /dev/null ; then
     # ci-fairy not available. Cannot check.
     exit 0
 fi