Allow external PRRTE in OpenMPI easyblock

[bartoldeman]

(created using eb --new-pr)

[bedroge]

Test report by @bedroge

Overview of tested easyconfigs (in order)

• SUCCESS OpenMPI-4.1.5-GCC-12.2.0.eb
• SUCCESS OpenMPI-5.0.3-GCC-13.3.0.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total)
bob-Latitude-5300 - Linux Ubuntu 22.04, x86_64, Intel(R) Core(TM) i7-8665U CPU @ 1.90GHz, Python 3.10.12
See https://gist.github.com/bedroge/8b1d2443d1746bf37a798218923f741e for a full test report.

This was using the old version of the OpenMPI 5.0.3 easyconfig, i.e. without the changes from easybuilders/easybuild-easyconfigs#20698. The log shows that it used --with-prrte=internal, as expected/intended. The test reports in the easyconfigs PR show that it picks up the right PRRTE when it's listed as dependency in the easyconfig.

[branfosj]

Going in, thanks @bartoldeman!