improve PR comment message to let people install the built extension

Signed-off-by: Guillaume Tardif <guillaume.tardif@gmail.com>

Author: gtardif

.github/workflows/build-scan-push.yaml

@@ -152,6 +152,10 @@ jobs:
             Docker image tag(s) pushed:
             ```text
             ${{ steps.docker_meta.outputs.tags }}
+            ```
+             To install the extension from this PR:
+            ```text
+            docker extension install -f ${{ steps.docker_meta.outputs.tags }}
             ```
             Labels added to images:
             ```text