build: run ShellCheck in CI (#31809)

build: run ShellCheck

Adds a ShellCheck check to edx-platform PRs and master,
using the shared workflow & template from the .github repo.
This will become a "required" check once it passes for 2 straight weeks on master.

Brings all existing shell scripts into compliance with ShellCheck.

scripts/all-tests.sh

@@ -13,13 +13,13 @@ set -e
 # Violations thresholds for failing the build
 source scripts/thresholds.sh
 
-XSSLINT_THRESHOLDS=`cat scripts/xsslint_thresholds.json`
+XSSLINT_THRESHOLDS=$(cat scripts/xsslint_thresholds.json)
 export XSSLINT_THRESHOLDS=${XSSLINT_THRESHOLDS//[[:space:]]/}
 
 
-# Run appropriate CI system script
-if [ -n "$SCRIPT_TO_RUN" ] ; then
-    $SCRIPT_TO_RUN
+# Run appropriate CI system script (with args if provided)
+if [ -n "${SCRIPT_TO_RUN[*]}" ] ; then
+    "${SCRIPT_TO_RUN[@]}"
 
     # Exit with the exit code of the called script
     exit $?