Bypass executable check when creating RunSettings

[ashao]

In some instances, the strong check that the executable can be found by the driver script when creating RunSettings is too restrictive. This can occur in cases where the binary does not exist on the node where the driver script is being launched (e.g. remote execution) or for binaries which need to be found after setting environment variables.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.72%. Comparing base (d7d979e) to head (a5e53d0).
Report is 19 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #786      +/-   ##
===========================================
- Coverage    83.91%   81.72%   -2.19%     
===========================================
  Files           83       84       +1     
  Lines         6284     7011     +727     
===========================================
+ Hits          5273     5730     +457     
- Misses        1011     1281     +270     

Files with missing lines	Coverage Δ
smartsim/settings/base.py	90.73% <100.00%> (-7.46%)	⬇️

... and 46 files with indirect coverage changes

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[al-rigazzi]

Could you please add a test -- or even parameterize an existing one, so that we can check for correct behavior when skip_exe_check==True?
Because otherwise I agree that this is too restrictive and this can be merged pretty soon!