Fix data race on desiredWorkflowCacheSize

[nghiant3223]

What was changed

Add lock/unlock to desiredWorkflowCacheSize in func NewWorkerCache() *WorkerCache.

Why?

Prevent data race.

Checklist

1. Closes Data race due to missing lock/unlock #1375

2. How was this tested: Run the test in ticket description to verify.

3. Any docs updates needed? No

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[cretz]

LGTM, will let @Quinn-With-Two-Ns weigh in.

As mentioned on the other PR, if you are hitting this race, it means whatever you're doing is trying to update a cache variable that is already being used in its one spot, so you have bigger problems. Cache size should only be set before everything.

[Quinn-With-Two-Ns]

+1 to everything @cretz said