Change antrea-image-platform input to 'choice'

Signed-off-by: Antonin Bas <[email protected]>

.github/workflows/conformance.yml

@@ -40,7 +40,13 @@ on:
         default: ubuntu-latest
       antrea-image-platform:
         description: Platform argument to provide when building the Antrea images
+        type: choice
+        options:
+          - "linux/amd64"
+          - "linux/arm64"
+          - "linux/arm/v7"
         default: "linux/amd64"
+        required: true
 
 
 jobs: