ci/shellcheck: Don't exit on first failure

It's really tedious having to run shellcheck in a loop to find every
failure; go through them all and print them all at once.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31602>

.gitlab-ci/run-shellcheck.sh

@@ -9,15 +9,14 @@ is_bash() {
     return 1
 }
 
+anyfailed=0
+
 while IFS= read -r -d $'' file; do
     if is_bash "$file" ; then
-        shellcheck -x -W0 -s bash "$file"
-        rc=$?
-        if [ "${rc}" -eq 0 ]
-        then
-            continue
-        else
-            exit 1
+        if ! shellcheck -x -W0 -s bash "$file"; then
+            anyfailed=1
         fi
     fi
 done < <(find "$SCRIPTS_DIR" -type f \! -path "./.git/*" -print0)
+
+exit "$anyfailed"