Add ability to set git auth token using environment variables

[yorinasub17]

Description

Currently the only way to set the git auth is by including it in the context git URL. This can be problematic for certain environments such as ECS where the command args are stored in logs. This changeset introduces updates to allow setting the authentication info using the environment variables GIT_USERNAME and GIT_PASSWORD.

Submitter Checklist

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review them:

• Includes unit tests
• Adds integration tests if needed.

See the contribution guide for more details.

NOTE: I didn't add an integration test for this change given that it requires real git credentials with a private git repo to fully test. If the maintainers have ideas for how to do this, happy to attempt an implementation. FWIW, I ran this manually after creating a new container and it worked perfectly:

%~> docker run \                                                                                                                                  
  -e GIT_PULL_METHOD=https -e GITHUB_OAUTH_TOKEN -e GIT_USERNAME \
   kaniko:git-auth-token-env \
   --context git://github.com/gruntwork-io/module-ci.git#refs/heads/master \
   --context-sub-path modules/ecs-deploy-runner/docker --no-push \
   --build-arg GITHUB_OAUTH_TOKEN
Enumerating objects: 291, done.
Counting objects: 100% (291/291), done.
Compressing objects: 100% (170/170), done.
Total 5678 (delta 178), reused 196 (delta 112), pack-reused 5387
INFO[0004] Resolved base name python:3.8-alpine to with-secrets
INFO[0004] Resolved base name python:3.8-alpine to without-secrets

Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

- For git contexts, you can now set authentication credentials for basic auth using environment variables `GIT_USERNAME` and `GIT_PASSWORD`.

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[yorinasub17]

@googlebot I signed it!

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[tejal29]

Thank you for your PR!
One questions about usability.