add missing return for customised install_extension_async methods in Rserve and Rmpi easyblocks

[boegel]

This is requires in order to be able to use --parallel-extensions-install on R-bundle-CRAN-2024.06-foss-2023b.eb, without it we get a hard crash:

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 "/tmp/vsc40023/easybuild-framework/easybuild/main.py", line 801, in <module>
    main_with_hooks()
  File "/tmp/vsc40023/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 "/tmp/vsc40023/easybuild-framework/easybuild/main.py", line 743, in main
    hooks, do_build)
  File "/tmp/vsc40023/easybuild-framework/easybuild/main.py", line 567, in process_eb_args
    exit_on_failure=exit_on_failure)
  File "/tmp/vsc40023/easybuild-framework/easybuild/main.py", line 175, in build_and_install_software
    raise ec_res['err']
  File "/tmp/vsc40023/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 "/tmp/vsc40023/easybuild-framework/easybuild/framework/easyblock.py", line 4373, in build_and_install_one
    result = app.run_all_steps(run_test_cases=run_test_cases)
  File "/tmp/vsc40023/easybuild-framework/easybuild/framework/easyblock.py", line 4241, in run_all_steps
    self.run_step(step_name, step_methods)
  File "/tmp/vsc40023/easybuild-framework/easybuild/framework/easyblock.py", line 4084, in run_step
    step_method(self)()
  File "/tmp/vsc40023/easybuild-framework/easybuild/framework/easyblock.py", line 2973, in extensions_step
    self.install_all_extensions(install=install)
  File "/tmp/vsc40023/easybuild-framework/easybuild/framework/easyblock.py", line 1939, in install_all_extensions
    self.install_extensions_parallel(install=install)
  File "/tmp/vsc40023/easybuild-framework/easybuild/framework/easyblock.py", line 2059, in install_extensions_parallel
    if self.dry_run or ext.async_cmd_task.done():
AttributeError: 'NoneType' object has no attribute 'done'

[Micket]

lgtm