fix: data race for isStarted

[rfyiamcool]

change

if we run go test -race ... or go run -race, raise data race in some case.

[codecov-commenter]

Codecov Report

Merging #65 (68ebafd) into master (485d76b) will decrease coverage by 0.35%.
Report is 2 commits behind head on master.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
- Coverage   90.13%   89.79%   -0.35%     
==========================================
  Files           8        8              
  Lines         527      529       +2     
==========================================
  Hits          475      475              
- Misses         33       35       +2     
  Partials       19       19              

Files Changed	Coverage Δ
quartz/scheduler.go	91.80% <0.00%> (-1.02%)	⬇️