github: backend-test: Skip coverage steps unless PR

Because they are not used on branch builds like on main.

Signed-off-by: René Dudfield <renedudfield@microsoft.com>

.github/workflows/backend-test.yml

@@ -56,6 +56,7 @@ jobs:
         shell: bash
 
       - name: Get base branch code coverage
+        if: ${{ github.event_name }} == 'pull_request'
         run: |
           cd backend
           base_branch="${{ github.base_ref }}"
@@ -69,6 +70,7 @@ jobs:
         shell: bash
 
       - name: Compare code coverage
+        if: ${{ github.event_name }} == 'pull_request'
         run: |
           testcoverage="${{ env.coverage }}"
           base_coverage="${{ env.base_coverage }}"
@@ -87,6 +89,7 @@ jobs:
         shell: bash
 
       - name: Comment on PR
+        if: ${{ github.event_name }} == 'pull_request'
         run: |
           testcoverage="${{ env.coverage }}"
           base_coverage="${{ env.base_coverage }}"