replace run_cmd with run_shell_cmd in custom easyblock for MrBayes (mrbayes.py)

[branfosj]

(created using eb --new-pr)

• no use of EB_MrBayes in easyconfigs or blocks
• block is only used for <= 3.2.6

[branfosj]

Test report by @branfosj

Overview of tested easyconfigs (in order)

• SUCCESS pocl-1.6-GCC-10.2.0.eb
• SUCCESS beagle-lib-3.1.2-GCC-10.2.0.eb
• SUCCESS MrBayes-3.2.6-gompi-2020b.eb

Build succeeded for 3 out of 3 (1 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/2474a88474c27f2a1bbbd2de197c3d88 for a full test report.

[bartoldeman]

LGTM