Remove multi-arch for pull request builds

It makes the PR build faster while preserving
multi-arch for the pushed image.

Signed-off-by: Dale Haiducek <[email protected]>

Author: dhaiducek

.tekton/cluster-api-provider-agent-mce-29-pull-request.yaml

@@ -29,9 +29,6 @@ spec:
     - name: build-platforms
       value:
         - linux/x86_64
-        - linux/ppc64le
-        - linux/s390x
-        - linux/arm64
     - name: dockerfile
       value: Dockerfile.rhtap
     - name: path-context