fix: Add Git Username env variable #4315

[thiha-min-thant]

Proposed changes

• Added support for setting the Git username as an environment variable (GIT_USERNAME) to ensure compatibility with Bitbucket repositories.
• This change addresses an issue where Bitbucket requires the username explicitly, and the default system username did not work correctly for authentication.
• Fixed Unable to import a litmushub from bitbucket(private repository) #4315

Types of changes

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• New feature (non-breaking change which adds functionality)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices applies)

Checklist

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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Dependency

• Please add the links to the dependent PR need to be merged before this (if any).

Special notes for your reviewer:

[amityt]

LGTM! Thanks for the fix @thiha-min-thant 🙌🏼

[Saranya-jena]

lgtm