promotion-diff: ignore COMPONENT, not just STREAM/DESCRIPTION

build-args.conf gained a COMPONENT line, set per-stream like STREAM and
DESCRIPTION (e.g. fedora-coreos-testing-devel vs fedora-coreos-testing),
so we need to ignore COMPONENT as well.

.github/workflows/promotion-diff.yml

@@ -46,7 +46,7 @@ jobs:
       - name: Normalize build-args.conf
         run: |
           # Ignore those args differences between streams since it's stream-specific.
-          for arg in DESCRIPTION STREAM; do
+          for arg in DESCRIPTION STREAM COMPONENT; do
               sed -i "/^${arg}=/d" origin/build-args.conf new/build-args.conf
           done
       - name: Compare trees