Change to use new base images for git and pullrequest images

[zhangtbj]

This PR is a follow-up PR after #3810

We already swtiched the git-init and pullrequest-init` Dockerfile into the different folder and add these two image build process in the build/publish process.

In this PR, we can officially switch to use these two build-base image in the .ko.yaml and remove the old Dockerfile or build-base image.

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Add nonroot user in the PullRequest init base image

[zhangtbj]

/kind misc
/ok-to-test

[zhangtbj]

/retest

[zhangtbj]

Seem Tekton still cannot find the git-init-build-base and pullrequest-init-build-base images in the internal flow.... :(

[zhangtbj]

Also cc @sbwsg ^^

[afrittoli]

/test pull-tekton-pipeline-build-tests

[afrittoli]

/test pull-tekton-pipeline-integration-tests

[afrittoli]

The images were still not found because the build was broken - fixed in #3834

[afrittoli]

Thank you!
/approve

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[ghost]

Awesome, thanks @zhangtbj and @afrittoli !!

/lgtm

[zhangtbj]

Also thank you @afrittoli and @sbwsg ! :)