bump vm-builder version to v0.15.0-alpha1

[fprasx]

Problem

Summary of changes

This is the second time bumping the version to v0.15.0-alpha1. The previous time there was an issue with the monitor that was hotfixed. We reran the release workflow to produce new images, even though the release version is the same.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

1264 tests run: 1211 passed, 0 failed, 53 skipped (full report)

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Flaky tests (2)

Postgres 14

• test_ondemand_download_large_rel[real_s3]: debug
• test_get_tenant_size_with_multiple_branches: debug