replace run_cmd with run_shell_cmd in custom easyblock for pybind11 (pybind11.py)

[bartoldeman]

(created using eb --new-pr)

[branfosj]

Test report by @branfosj

Overview of tested easyconfigs (in order)

• SUCCESS pybind11-2.6.0-GCCcore-10.2.0.eb
• SUCCESS pybind11-2.6.2-GCCcore-10.3.0.eb
• SUCCESS pybind11-2.7.1-GCCcore-11.2.0.eb
• SUCCESS pybind11-2.7.1-GCCcore-11.2.0-Python-2.7.18.eb
• SUCCESS pybind11-2.9.2-GCCcore-11.3.0.eb
• SUCCESS pybind11-2.11.1-GCCcore-13.2.0.eb
• SUCCESS Catch2-2.13.9-GCCcore-12.3.0.eb
• SUCCESS Catch2-2.13.9-GCCcore-12.2.0.eb
• SUCCESS pybind11-2.11.1-GCCcore-12.3.0.eb
• SUCCESS pybind11-2.10.3-GCCcore-12.2.0.eb

Build succeeded for 10 out of 10 (8 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/6ce2a5dabc813de865fe36ccc6a13bb5 for a full test report.

[branfosj]

Going in, thanks @bartoldeman!