Resolve FileDownloader.get_os_version exception for missing lsb_release

[jsiirola]

Fixes IDAES/idaes-pse#1221

Summary/Motivation:

FileDownloader.get_os_version() will generate FileNotFoundError exceptions on Linux platforms that are missing lsb_release in the PATH. This resolves that exception by first checking for the existence of the executable using shutil.which() (the approach recommended for locating executables in the subprocess.Popen documentation)

Changes proposed in this PR:

• use shutil.which to locate / check for the existence of lsb_release

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[mrmundt]

This looks good to me, but something is clearly up with the testing infrastructure today.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (4f2d5fc) 87.82% compared to head (d844730) 87.83%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2911      +/-   ##
==========================================
+ Coverage   87.82%   87.83%   +0.01%     
==========================================
  Files         770      770              
  Lines       89637    89639       +2     
==========================================
+ Hits        78726    78737      +11     
+ Misses      10911    10902       -9     

Flag	Coverage Δ
linux	84.89% <100.00%> (+<0.01%)	⬆️
osx	74.63% <33.33%> (+<0.01%)	⬆️
other	85.07% <100.00%> (-0.01%)	⬇️
win	81.97% <33.33%> (-0.19%)	⬇️

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Files Changed	Coverage Δ
pyomo/common/download.py	88.36% <100.00%> (+0.10%)	⬆️

... and 2 files with indirect coverage changes

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