Prevent analyzers from being excluded in scan execution pipelines

What does this MR do and why?

This MR fixes an oversight where it was possible to exclude certain analyzers from being run in scan execution pipelines.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Add .gitlab-ci.yml:

   build-job:
     stage: build
     script:
     - echo "Compiling the code..."
     - echo "Compile complete."
   include:
   - template: Security/SAST.gitlab-ci.yml

2. Add run_os_script.rb file into the project:

   class Generic
     def run_on_system
       system("ls")
     end
   end

3. Go to Secure -> Policies and create a new Scan Execution policy

   name: SAST
   description: ''
   enabled: true
   actions:
   - scan: sast
   rules:
   - type: pipeline
     branches:
     - "*"

4. Go to Settings -> CI/CD -> Variables and add a new variable:
5. Go to Pipelines and run a new pipeline for the main branch
6. Verify that brakeman-sast-0 job is still present in the pipeline

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #420700 (closed)