Add some branding

This follows the guidance provided in
https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#branding

Sadly, we cannot override the icon, only companies partnering with
GitHub can do that:
https://github.community/t/can-one-set-a-custom-image-icon-for-a-github-action-or-github-workflow-or-only-github-apps/17513

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

action.yml

@@ -1,6 +1,9 @@
 name: 'Setup Git for Windows SDK'
 description: 'Set up an environment to develop Git for Windows'
 author: 'Johannes Schindelin'
+branding:
+  icon: fast-forward
+  color: blue
 inputs:
   flavor:
     required: false