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

[QuantumNeuralCoder]

Issue # (if applicable)

Closes #.

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

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Checklist

• 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

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