CI: fix 'Check and Display Regression Diffs' step to use bash shell (#1673)

* CI: fix 'Check and Display Regression Diffs' step to use bash shell

The step used bash-specific [[ ]] syntax but lacked shell: bash {0},
causing failures when the runner defaulted to sh.

Author: yjhjstz

.github/workflows/build-cloudberry-rocky8.yml

@@ -1676,6 +1676,7 @@ jobs:
 
       - name: Check and Display Regression Diffs
         if: always()
+        shell: bash {0}
         run: |
           # Search for regression.diffs recursively
           found_file=$(find . -type f -name "regression.diffs" | head -n 1)

.github/workflows/build-cloudberry.yml

@@ -1679,6 +1679,7 @@ jobs:
 
       - name: Check and Display Regression Diffs
         if: always()
+        shell: bash {0}
         run: |
           # Search for regression.diffs recursively
           found_file=$(find . -type f -name "regression.diffs" | head -n 1)

.github/workflows/build-deb-cloudberry.yml

@@ -1618,6 +1618,7 @@ jobs:
 
       - name: Check and Display Regression Diffs
         if: always()
+        shell: bash {0}
         run: |
           # Search for regression.diffs recursively
           found_file=$(find . -type f -name "regression.diffs" | head -n 1)