fix(workflow/security-guardian): resolves bad object error when its run in the context of main branch

[QuantumNeuralCoder]

Issue # (if applicable)

Github action security-guradian is unable to detect changed cfn templates in PRs
Closes #.
NA

Reason for this change

Fixes an issue where security-guardian github action runs but is unable to find changed templates. Since it runs in the context of pull_request_target workflow its unable to find the head commit of PR which is in the base branch. This happened after we switched from pull_request to pull_request_target for improved security posture during the run of the github action.

Run echo "Getting changed CloudFormation templates..."
Getting changed CloudFormation templates...
From https://github.com/aws/aws-cdk
 * branch                  main       -> FETCH_HEAD
fatal: bad object 7c12c04a9d7bde97dda3caec8e3fcf7102f2f938

Description of changes

Checksout and compares the base version with the head version.

Describe any new or updated permissions being added

None

Description of how you validated changes

Cannot be validated until merged

Checklist

• [ x] My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

The pull request linter fails with the following errors:

❌ Fixes must contain a change to a test file.
❌ Fixes must contain a change to an integration test file and the resulting snapshot.
❌ The title prefix of this pull request must be one of "feat|fix|build|chore|ci|docs|style|refactor|perf|test|revert"

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[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 4ca9349
• Result: SUCCEEDED
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