Include main in branches for Azure nightly job

[AverageMarcus]

What this PR does / why we need it:

Includes the main branch in the triggers for the Azure nightly build job in preparation of renaming the master branch.

Which issue(s) this PR fixes (optional, in fixes #(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Towards: #1161

Additional context
Add any other context for the reviewers

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AverageMarcus

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[jsturtevant]

/lgtm
/hold for @mboersma, we might require some changes on our internal pipelines

[mboersma]

/lgtm
/hold cancel

I don't think we have any other triggered pipelines, although we will definitely have to play some catch-up once the branch is actually renamed.

[AverageMarcus]

/test ?

[k8s-ci-robot]

@AverageMarcus: The following commands are available to trigger required jobs:

• /test json-sort-check
• /test pull-azure-vhds
• /test pull-goss-populate
• /test pull-ova-all
• /test pull-packer-validate

The following commands are available to trigger optional jobs:

• /test pull-azure-sigs
• /test pull-container-image-build
• /test pull-image-builder-gcp-all

Use /test all to run the following jobs that were automatically triggered:

• pull-azure-sigs
• pull-azure-vhds
• pull-packer-validate

In response to this:

/test ?

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[AverageMarcus]

/test pull-azure-vhds