ci: bump (and pin) rust-cache action to v2.7.3

[Br1ght0ne]

After pushing #1467, I noticed that the CI hangs on saving rust-cache for ~3 minutes. After a quick look, seems like it's a known issue and fixed in the action's v2.7.3 version.

Our ci.yml has two uses steps with rust-cache: setup-test-projects using v2 (which was using the latest version automatically, and thus never experienced this problem), and cargo-verifications using v2.0.1 (severely outdated now).

Now both steps use a pinned v2.7.3 version, which fixes the saving hangup issue. Pinning should avoid such problems in the future.

Checklist

• [x] I have linked to any relevant issues.
• [x] I have added necessary labels.
• [x] I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• [x] I have requested a review from the relevant team or maintainers.