core: fix data race in scheduler

[xenowits]

Fixes data race in scheduler.go.

category: fixbuild
ticket: none

[codecov]

Codecov Report

Patch coverage: 50.00% and project coverage change: +0.04 🎉

Comparison is base (02338c7) 55.33% compared to head (3617536) 55.37%.

❗ Current head 3617536 differs from pull request most recent head 6148089. Consider uploading reports for the commit 6148089 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1866      +/-   ##
==========================================
+ Coverage   55.33%   55.37%   +0.04%     
==========================================
  Files         169      169              
  Lines       21825    21825              
==========================================
+ Hits        12076    12085       +9     
+ Misses       8193     8187       -6     
+ Partials     1556     1553       -3     

Impacted Files	Coverage Δ
core/scheduler/scheduler.go	77.34% <50.00%> (ø)
core/leadercast/transport.go	75.14% <0.00%> (-1.19%)	⬇️
core/qbft/qbft.go	82.98% <0.00%> (-0.85%)	⬇️
app/app.go	64.18% <0.00%> (+0.33%)	⬆️
core/priority/prioritiser.go	61.79% <0.00%> (+1.41%)	⬆️
app/vmock.go	77.94% <0.00%> (+5.12%)	⬆️

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