fix IsExecuting and Can Execute threading issue

[ChrisPulman]

What kind of change does this PR introduce?

fix #2834

What is the current behaviour?

Executing fails if a TaskPool thread is selected

What is the new behaviour?

Executing works if a TaskPool thread is selected

What might this PR break?

none expected

Please check if the PR fulfils these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

[codecov]

Codecov Report

Merging #2835 (b849692) into main (aaacea8) will increase coverage by 0.30%.
The diff coverage is 81.63%.

@@            Coverage Diff             @@
##             main    #2835      +/-   ##
==========================================
+ Coverage   71.50%   71.81%   +0.30%     
==========================================
  Files         137      137              
  Lines        4804     4828      +24     
==========================================
+ Hits         3435     3467      +32     
+ Misses       1369     1361       -8     

Impacted Files	Coverage Δ
.../ReactiveUI/ReactiveCommand/ReactiveCommandBase.cs	100.00% <ø> (ø)
src/ReactiveUI/ReactiveCommand/ReactiveCommand.cs	82.07% <80.48%> (+1.24%)	⬆️
...ctiveUI/ReactiveCommand/CombinedReactiveCommand.cs	81.35% <87.50%> (-0.13%)	⬇️
src/ReactiveUI/RxApp.cs	75.34% <0.00%> (+2.73%)	⬆️
...veUI/Platforms/net5/ComponentModelTypeConverter.cs	88.88% <0.00%> (+4.44%)	⬆️
...atforms/netcoreapp3/ComponentModelTypeConverter.cs	88.88% <0.00%> (+4.44%)	⬆️
...veUI/Platforms/net4/ComponentModelTypeConverter.cs	86.11% <0.00%> (+5.55%)	⬆️
...UI/Platforms/windows-common/ReactiveUserControl.cs	90.90% <0.00%> (+18.18%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update aaacea8...b849692. Read the comment docs.

[glennawatson]

Change to be configurable

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.