fix(ci): Switch CI to a GCP zone with available instances

[teor2345]

Motivation

GCP CI fails with:

A c2d-standard-16 VM instance is currently unavailable in the us-central1-a
zone. Consider trying your request in the us-central1-f, us-central1-b, us-central1-c
zone(s), which currently has capacity to accommodate your request. Alternatively,
you can try your request again with a different VM hardware configuration or
at a later time. For more information, see the troubleshooting documentation.

https://github.com/ZcashFoundation/zebra/actions/runs/4429186789/jobs/7772459849#step:10:94

Solution

• Switch the only workflow that uses c2d-standard-16 machines to us-central1-f

Review

This is an urgent CI failure fix.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

Follow Up Work

We'll also need to change the corresponding variable in PR #6357.

[teor2345]

The same error happened in:
https://github.com/ZcashFoundation/zebra/actions/runs/4429186789/jobs/7772459124#step:10:94
https://github.com/ZcashFoundation/zebra/actions/runs/4429186789/jobs/7772459672#step:10:94
https://github.com/ZcashFoundation/zebra/actions/runs/4427938683/jobs/7772461002#step:10:94

[gustavovalverde]

LGTM