ci: Fix creating GCP instances

[upbqdn]

Motivation

• Close main branch CI failed: push in Integration Tests on GCP #8151.
• We're about to release Zebra 1.6, and a necessary precondition is to: "Make sure there has been at least one successful full sync test since the last state change, [...]", and there's no such test.

PR Author Checklist

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• Will the PR name make sense to users?
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Solution

• Remove the DISK_OPTION env var, which causes this error: https://github.com/ZcashFoundation/zebra/actions/runs/7930252459/job/21652230260#step:11:76. The problem is that the variable is set in another job with id get-disk-name, and evaluates to an empty string in other jobs.

Reviewer Checklist

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• Does the PR scope match the ticket?
• Are there enough tests to make sure it works? Do the tests cover the PR motivation?
• Are all the PR blockers dealt with?
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And check the PR Author checklist is complete.

Follow Up Work

Refactor the cached disk selection. TODO: Replace the previous sentence with an issue number.

[upbqdn]

Removing the variable effectively turns off using cached disks, but I don't think that worked before because we had a syntactic error in the --create-disk flag. We were missing a comma, which was added in 516fe1c (#8005). Adding the comma likely started causing the current issue since the --create-disk flag now evaluates to --create-disk=,, which seems to be the bug. Without the comma, DISK_OPTION evaluated to an empty string, and the name property directly followed, so the --create-disk flag was syntactically correct, but the image=$CACHED_DISK_NAME propery, set in DISK_OPTION, was missing.

Edit: the above is not quite right. I opened #8302.

[upbqdn]

This PR didn't address the underlying issue, so I'm closing it. We should address #8302 instead.