inject custom '%(python)s' template value before getting value of 'buildcmd' custom easyconfig parameter in PythonPackage easyblock

[boegel]

Fix for crash when installing PySide2-5.14.2.3-GCCcore-10.2.0.eb:

Traceback (most recent call last):
  File "/usr/lib64/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib64/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/main.py", line 801, in <module>
    main_with_hooks()
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/main.py", line 787, in main_with_hooks
    main(args=args, prepared_cfg_data=(init_session_state, eb_go, cfg_settings))
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/main.py", line 743, in main
    hooks, do_build)
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/main.py", line 567, in process_eb_args
    exit_on_failure=exit_on_failure)
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/main.py", line 175, in build_and_install_software
    raise ec_res['err']
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/main.py", line 136, in build_and_install_software
    (ec_res['success'], app_log, err_msg, err_code) = build_and_install_one(ec, init_env)
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/framework/easyblock.py", line 4461, in build_and_install_one
    result = app.run_all_steps(run_test_cases=run_test_cases)
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/framework/easyblock.py", line 4329, in run_all_steps
    self.run_step(step_name, step_methods)
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-framework/easybuild/framework/easyblock.py", line 4172, in run_step
    step_method(self)()
  File "/user/gent/400/vsc40023/easybuild/eb5/easybuild-easyblocks/easybuild/easyblocks/generic/pythonpackage.py", line 835, in build_step
    build_cmd = build_cmd % {'python': self.python_cmd}
KeyError: 'parallel'

The tricky thing here is that while the buildcmd value may contain general template values (like %(parallel)s), there's also this custom %(python)s that is used locally here in PythonPackage.build_step.
The only proper way of correctly handling any possible combination of these cases at this point is by injecting the value for the %(python)s template in the dictionary of template values (which shouldn't cause trouble anywhere).

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS PySide2-5.14.2.3-GCCcore-10.2.0.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
node3509.doduo.os - Linux RHEL 8.8, x86_64, AMD EPYC 7552 48-Core Processor, Python 3.6.8
See https://gist.github.com/boegel/079c47d10614bf1af84493aec01f6ed3 for a full test report.

[lexming]

LGTM

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS PySide2-5.14.2.3-GCCcore-10.2.0.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
node3509.doduo.os - Linux RHEL 8.8, x86_64, AMD EPYC 7552 48-Core Processor, Python 3.6.8
See https://gist.github.com/boegel/5f57495c82101a4afcafe64683cde4ba for a full test report.