ci: add support for a prepare step before using the cache

[remyleone]

Inspiration: rclone/rclone@8b981d8

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.73%. Comparing base (306abf0) to head (c8a2b5f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6045   +/-   ##
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  Coverage   50.73%   50.73%           
=======================================
  Files         359      359           
  Lines       82139    82139           
=======================================
  Hits        41674    41674           
  Misses      40465    40465           

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[Copilot]

Pull request overview

This PR updates the CI test workflow to introduce a dedicated “prepare” job that pre-populates the Go module cache before running the main test jobs, aiming to reduce repeated dependency downloads across the matrix.

Changes:

• Adds a new prepare matrix job (ubuntu/macos/windows) that downloads Go modules and populates a shared module cache.
• Makes namespaces and others depend on prepare and restores the module cache before running tests.
• Disables setup-go built-in caching to rely on the explicit cache steps.

Suppressed comments (2)

.github/workflows/tests.yml:44

• Because prepare is a matrix job, needs: prepare will make each namespaces run wait for all prepare matrix runs (ubuntu/macos/windows) to finish, even though each namespaces run only needs the cache for its own OS. This can increase the workflow critical path; consider splitting namespaces into per-OS jobs (each needing its corresponding prepare job) or avoiding the cross-job dependency by doing cache restore/update within the same job.

    needs: prepare

.github/workflows/tests.yml:121

• Same matrix-wide dependency concern here: needs: prepare forces others to wait for all three OS variants of the prepare job, even though others only runs on a single runner. If the goal is to speed up CI, consider removing this dependency or restructuring so others only waits on the relevant prepare job.

    needs: prepare

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