(gha) Call shellcheck with -x so it will load external files

Will allow the shellcheck test of task files to source common shell
scripts in files/ when testing.

Author: jpartlow

.github/workflows/pr_testing.yaml

@@ -17,7 +17,7 @@ jobs:
       - name: Install ShellCheck
         run: sudo apt-get install shellcheck
       - name: Run ShellCheck
-        run: shellcheck tasks/*.sh
+        run: shellcheck -x tasks/*.sh
 
   test-install-task-on-ubuntu:
     strategy: