github: style check on diff only for this repo

This repository is not yet fully style-clean, and the policy is to
update files as we touch them, not in one big go. Until we have reached
a fully style-clean state, we should check only the diff on push, not
all files.

Signed-off-by: Gerwin Klein <gerwin.klein@proofcraft.systems>

.github/workflows/push.yml

diff --git a/.github/workflows/push.yml b/.github/workflows/push.yml
index 31da3cd..76dbb0a 100644
--- a/.github/workflows/push.yml
+++ b/.github/workflows/push.yml
@@ -31,3 +31,5 @@
     runs-on: ubuntu-latest
     steps:
     - uses: seL4/ci-actions/style@master
+      with:
+        diff_only: true