replace run_cmd with run_shell_cmd in custom easyblock for NEURON (neuron.py)

[branfosj]

(created using eb --new-pr)

• no use of EB_NEURON in easyconfigs or blocks

[branfosj]

Test report by @branfosj

Overview of tested easyconfigs (in order)

• SUCCESS NEURON-7.8.2-foss-2021b.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
bear-pg0105u03b - Linux RHEL 8.6, x86_64, Intel(R) Xeon(R) Platinum 8360Y CPU @ 2.40GHz (icelake), Python 3.6.8
See https://gist.github.com/branfosj/9404529b8079e9f9de7c56e14e936cf4 for a full test report.

[bartoldeman]

LGTM