replace run_cmd with run_shell_cmd in custom easyblock for Nim (nim.py)

[branfosj]

(created using eb --new-pr)

• no use of EB_Nim in easyconfigs or blocks

[branfosj]

Test report by @branfosj

Overview of tested easyconfigs (in order)

• SUCCESS Nim-1.4.8-GCCcore-10.3.0.eb
• SUCCESS Nim-1.6.6-GCCcore-11.2.0.eb

Build succeeded for 2 out of 2 (2 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/20b14c7b98638ae0b62ccb0cc423cee6 for a full test report.

[bartoldeman]

LGTM