replace run_cmd with run_shell_cmd in custom easyblock for ParMETIS (parmetis.py)

[bartoldeman]

(created using eb --new-pr)

[branfosj]

Test report by @branfosj

Overview of tested easyconfigs (in order)

• SUCCESS ParMETIS-4.0.3-gompi-2020b.eb
• SUCCESS ParMETIS-4.0.3-gompi-2021a.eb
• SUCCESS ParMETIS-4.0.3-gompi-2021b.eb
• SUCCESS ParMETIS-4.0.3-gompi-2022a.eb
• SUCCESS ParMETIS-4.0.3-gompi-2022b.eb
• SUCCESS ParMETIS-4.0.3-iimpi-2021a.eb
• SUCCESS ParMETIS-4.0.3-iimpi-2021b.eb

Build succeeded for 7 out of 7 (7 easyconfigs in total)
bear-pg0105u03a - 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/74a84350a5fd96ede8460ed99cd53ddd for a full test report.

[branfosj]

Going in, thanks @bartoldeman!