Add missing shell instruction

Author: Donnerbart

merge-junit-reports/action.yml

@@ -27,6 +27,7 @@ runs:
   using: composite
   steps:
     - name: Set up xmlstarlet
+      shell: bash
       run: sudo apt update && sudo apt install -y xmlstarlet
 
     - name: Checkout JUnit reports
@@ -52,6 +53,7 @@ runs:
     - name: Clean up JUnit reports
       if: ${{ inputs.cleanup-junit-reports }}
       working-directory: junit-reports-artifact
+      shell: bash
       run: |
         echo -n "JUnit report size before cleanup: "
         du -sch *.xml | tail -n 1 | cut -f1
@@ -64,9 +66,11 @@ runs:
     - name: Delete old JUnit reports
       if: ${{ inputs.delete-old-junit-reports }}
       working-directory: junit-reports
+      shell: bash
       run: git rm *.xml || true
 
     - name: Copy new JUnit reports
+      shell: bash
       run: cp junit-reports-artifact/*.xml junit-reports/
 
     - name: Update JUnit reports