Limit parallel jobs on try/r+ runs

Rate limit of 60 applies org wide, let's not spam.
Thanks pietroalbini for the hint

Author: flip1995

.github/workflows/clippy_bors.yml

@@ -177,6 +177,7 @@ jobs:
     needs: changelog
     strategy:
       fail-fast: false
+      max-parallel: 6
       matrix:
         integration:
         - 'rust-lang/cargo'