fix: don't use backtick in error message

Author: marcoieni

src/ci/scripts/free-disk-space.sh

@@ -57,7 +57,7 @@ printSavedSpace() {
     local saved=$((after - before))
 
     if [ "$saved" -lt 0 ]; then
-        echo "::warning::Saved space is negative: $saved. Using `0` as saved space."
+        echo "::warning::Saved space is negative: $saved. Using '0' as saved space."
         saved=0
     fi