Disable interpolation of HEREDOC strings containing hook scripts

Interpolation was applied within the user-data.sh context, causing
unexpected empty values for variables and other unintentend effects.

While this behavior might be useful in certain cases, it requires
consistently escaping everything in hook scripts. This approach feels
error-prone and unintuitive.

For example, the following configuration will echo an empty string and
the date of string interpolation, but not the date of the job's start:
```
  runner_hook_job_started: |
    echo $GITHUB_WORKSPACE
    echo $(date)
```

Author: Peter-Sh

examples/multi-runner/templates/user-data.sh

@@ -82,14 +82,14 @@ ${post_install}
 cd /opt/actions-runner
 
 %{ if hook_job_started != "" }
-cat > /opt/actions-runner/hook_job_started.sh << EOF
+cat > /opt/actions-runner/hook_job_started.sh <<'EOF'
 ${hook_job_started}
 EOF
 echo ACTIONS_RUNNER_HOOK_JOB_STARTED=/opt/actions-runner/hook_job_started.sh | tee -a /opt/actions-runner/.env
 %{ endif }
 
 %{ if hook_job_completed != "" }
-cat > /opt/actions-runner/hook_job_completed.sh << EOF
+cat > /opt/actions-runner/hook_job_completed.sh <<'EOF'
 ${hook_job_completed}
 EOF
 echo ACTIONS_RUNNER_HOOK_JOB_COMPLETED=/opt/actions-runner/hook_job_completed.sh | tee -a /opt/actions-runner/.env

modules/runners/templates/user-data.sh

@@ -63,16 +63,16 @@ ${install_runner}
 ${post_install}
 
 # Register runner job hooks
-# Ref: https://github.com/actions/runner/blob/main/docs/adrs/1751-runner-job-hooks.md
+# Ref: https://docs.github.com/en/actions/hosting-your-own-runners/managing-self-hosted-runners/running-scripts-before-or-after-a-job
 %{ if hook_job_started != "" }
-cat > /opt/actions-runner/hook_job_started.sh << EOF
+cat > /opt/actions-runner/hook_job_started.sh <<'EOF'
 ${hook_job_started}
 EOF
 echo ACTIONS_RUNNER_HOOK_JOB_STARTED=/opt/actions-runner/hook_job_started.sh | tee -a /opt/actions-runner/.env
 %{ endif }
 
 %{ if hook_job_completed != "" }
-cat > /opt/actions-runner/hook_job_completed.sh << EOF
+cat > /opt/actions-runner/hook_job_completed.sh <<'EOF'
 ${hook_job_completed}
 EOF
 echo ACTIONS_RUNNER_HOOK_JOB_COMPLETED=/opt/actions-runner/hook_job_completed.sh | tee -a /opt/actions-runner/.env