fix default value branch name

.github/workflows/apptek_hashes.yml

@@ -21,7 +21,7 @@ jobs:
         python-version: 3.8
     - name: Start Bitbucket Pipeline
       run: |
-        curl ${{ env.post_header }} ${{ env.auth_token}} ${{ env.api_url }} -d '${{ env.post_content }}'${GITHUB_HEAD_REF:-main}'"}]}' |\
+        curl ${{ env.post_header }} ${{ env.auth_token}} ${{ env.api_url }} -d '${{ env.post_content }}'${GITHUB_HEAD_REF:-master}'"}]}' |\
         jq -r '.uuid' | sed 's/{/%7B/' | sed 's/}/%7D/' > pipeline_uuid.txt
     - name: Verify Start
       run: |
@@ -44,4 +44,4 @@ jobs:
             curl ${{ env.get_header }} ${{ env.auth_token}} ${{ env.api_url }}$(cat pipeline_uuid.txt) |\
             jq -r '.build_number') \
             "failed. Please contact {wmichel,ebeck}@apptek.com to find out why."
-        exit 1
+        exit 1