CI: Use FOSSA push-only token for license scans on PRs

[mattlord]

Description

Now that we have the FOSSA license scan working again on merges to main, let's enable the scan on PRs (opened using forks) again by leveraging a push-only token. You can see a successful run here: https://github.com/vitessio/vitess/actions/runs/7893049759/job/21540828304?pr=15222

This gets us back to where we were prior to having to disable it in #14119.

Related Issue(s)

• Hopefully the last follow-up to: Remove FOSSA Test from CI until we can do it in a secure way #14119

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

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