set "main" instead of "master" to indicate development branch

[Rakshith-R]

What type of PR is this?
/kind bug

What this PR does / why we need it:

The image creation currently fails since the script only considers "master" tag can be overwritten.
This commit replaces "master" with "main" to
indicate the development branch.

Which issue(s) this PR fixes:
Updates #74

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

Hi @Rakshith-R. Thanks for your PR.

I'm waiting for a kubernetes-csi member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[Rakshith-R]

/cc @PrasadG193 @carlbraganza

[carlbraganza]

/lgtm

[carlbraganza]

/ok-to-test

[carlbraganza]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: carlbraganza, Rakshith-R

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[carlbraganza]

/test pull-kubernetes-csi-external-snapshot-metadata-unit

[k8s-ci-robot]

@Rakshith-R: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kubernetes-csi-external-snapshot-metadata-unit	c07dd34	link	true	/test pull-kubernetes-csi-external-snapshot-metadata-unit

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.

[Rakshith-R]

I guess this needs to changed in https://github.com/kubernetes-csi/csi-release-tools/blob/master/build.make
And then merged into this repo.

I'll send a pr there tmr.