nrnversion_h: Use bash instead of $ENV{SHELL}

[alexsavulescu]

• use bash since we launch a bash script
• fixes Windows CI builds are broken - $ENV{SHELL} path expands Win-style #1590

[codecov-commenter]

Codecov Report

Merging #1591 (f06c3d3) into master (0050209) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1591      +/-   ##
==========================================
- Coverage   45.19%   45.15%   -0.04%     
==========================================
  Files         551      551              
  Lines      111216   111216              
==========================================
- Hits        50261    50217      -44     
- Misses      60955    60999      +44     

Impacted Files	Coverage Δ
src/nrnmpi/bbsmpipack.cpp	82.56% <0.00%> (-10.77%)	⬇️
src/parallel/bbsclimpi.cpp	50.00% <0.00%> (-8.14%)	⬇️
src/parallel/bbssrvmpi.cpp	42.16% <0.00%> (-6.03%)	⬇️
src/parallel/bbs.cpp	74.26% <0.00%> (-1.69%)	⬇️

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