chore(GHW): Smithy-Diff should be on the PR diff, not diff b/w main (#1070)

Author: texastony

.github/workflows/smithy-diff.yml

@@ -22,10 +22,11 @@ jobs:
         shell: bash
         env:
           GITHUB_HEAD: ${{github.event.pull_request.head.ref}}
+          GITHUB_BASE: ${{github.event.pull_request.base.ref}}
         run:
           # Checks to see if any of the smithy Models are being updated.
           # Doing this check allows us to catch things like, missing @javadoc trait documentation or bug in smithy dafny that has not be resolved.
-          echo "FILES=$(git diff --name-only origin/main origin/${GITHUB_HEAD} | grep '\.smithy$' | tr '\n' ' ')" >> "$GITHUB_OUTPUT"
+          echo "FILES=$(git diff --name-only origin/${GITHUB_BASE} origin/${GITHUB_HEAD} | grep '\.smithy$' | tr '\n' ' ')" >> "$GITHUB_OUTPUT"
 
       - name: Check if FILES is not empty
         id: comment