nrnivmodl_core_makefile: fix SDKROOT

[alexsavulescu]

Todo:

• test Azure drop with nrn-build-ci -> it works, however we no longer build on macOS-10.15. Also works locally.
• if successful, create CoreNeuron PR and then update this PR with official commit -> nrnivmodl_core_makefile: fix SDKROOT BlueBrain/CoreNeuron#845

[codecov-commenter]

Codecov Report

Merging #1942 (ba50afc) into master (fde3fc7) will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1942      +/-   ##
==========================================
- Coverage   47.37%   47.33%   -0.04%     
==========================================
  Files         543      543              
  Lines      112603   112603              
==========================================
- Hits        53351    53306      -45     
- Misses      59252    59297      +45     

Impacted Files	Coverage Δ
src/nrnmpi/bbsmpipack.cpp	81.95% <0.00%> (-10.25%)	⬇️
src/parallel/bbsclimpi.cpp	50.57% <0.00%> (-8.63%)	⬇️
src/parallel/bbssrvmpi.cpp	41.46% <0.00%> (-6.10%)	⬇️
src/parallel/bbs.cpp	74.57% <0.00%> (-1.70%)	⬇️

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[azure-pipelines]

✔️ b688ea334d8d81ab43116a106d04247a14157594 -> Azure artifacts URL

[azure-pipelines]

✔️ ba50afc -> Azure artifacts URL