Fix service nursery shielding when TaskStatus.started() is called from outside the new task

[oremanj]

Fixes python-trio/trio-asyncio#135

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d9226be) 100.00% compared to head (6af84fe) 100.00%.
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #27   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           15        15           
  Lines         1312      1328   +16     
  Branches       216       219    +3     
=========================================
+ Hits          1312      1328   +16     

Flag	Coverage Δ
3.10	99.92% <100.00%> (+<0.01%)	⬆️
3.11	99.92% <100.00%> (+<0.01%)	⬆️
3.12	99.92% <100.00%> (?)
3.12-dev	?
3.7	?
3.8	99.92% <100.00%> (+<0.01%)	⬆️
3.9	99.92% <100.00%> (+<0.01%)	⬆️
Ubuntu	99.92% <100.00%> (+<0.01%)	⬆️
Windows	99.92% <100.00%> (+<0.01%)	⬆️
macOS	99.92% <100.00%> (+<0.01%)	⬆️
pypy-3.8	99.32% <100.00%> (+<0.01%)	⬆️
pypy-3.9	99.32% <100.00%> (+<0.01%)	⬆️

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