Fix vsync race condition on providerscope unmount

[rrousselGit]

fixes #2022

[codecov]

Codecov Report

Merging #3072 (0dcdd4c) into master (66e7777) will decrease coverage by 0.21%.
The diff coverage is 28.57%.

❗ Current head 0dcdd4c differs from pull request most recent head 58032a1. Consider uploading reports for the commit 58032a1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3072      +/-   ##
==========================================
- Coverage   95.30%   95.10%   -0.21%     
==========================================
  Files          53       53              
  Lines        2281     2288       +7     
==========================================
+ Hits         2174     2176       +2     
- Misses        107      112       +5     

Files	Coverage Δ
packages/riverpod/lib/src/framework/scheduler.dart	87.23% <28.57%> (-10.27%)	⬇️