Make StartRotation concurrency safe

[sgress454]

Related issue: For #33111

Fixes a possible race condition introduced in #33884 which saw a test failure here.

Checklist for submitter

Testing

• Added/updated automated tests
  Existing test is sufficient, will circle back if it fails again.

• QA'd all new/changed functionality manually
  Verified that Fleet Desktop still opens My Device page correctly and token rotation logs are still seen.

For unreleased bug fixes in a release candidate, one of:

• Confirmed that the fix is not expected to adversely impact load test results

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.22%. Comparing base (dcefbc4) to head (ce303cb).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #34186      +/-   ##
==========================================
+ Coverage   64.21%   64.22%   +0.01%     
==========================================
  Files        2060     2060              
  Lines      207193   207279      +86     
  Branches     6895     6895              
==========================================
+ Hits       133046   133125      +79     
- Misses      63712    63717       +5     
- Partials    10435    10437       +2     

Flag	Coverage Δ
backend	65.35% <100.00%> (+0.01%)	⬆️

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[sgress454]

@lucasmrod Turns out I was not testing in race-condition mode like I thought, and once I fixed that it still failed because the actual issue causing the failure was in the test itself (although the patches to the rotation code are still needed!).

I patched the test using atomic and also fixed the name because rotater is apparently not a thing 🙃 . Verified with

go test -race -timeout 5m -tags full,fts5 -run ^TestRotator$ github.com/fleetdm/fleet/v4/orbit/pkg/token