fix pathsep join bug that breaks installation of CUDA versions < 10.1 if $PERL5LIB is defined

[Micket]

No description provided.

[boegel]

lgtm

[Micket]

Test report by @Micket

Overview of tested easyconfigs (in order)

• SUCCESS CUDA-9.2.88-GCC-7.3.0-2.30.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
vera-c1 - Linux centos linux 7.9.2009, x86_64, Intel Xeon Processor (Skylake), Python 3.6.8
See https://gist.github.com/d7fd9ab4a3a079cb81028a5bcbdb6f0f for a full test report.

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS CUDA-9.2.88.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
node2636.swalot.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz (haswell), Python 3.6.8
See https://gist.github.com/d61d1885016d6a3f382130a092466f46 for a full test report.

[boegel]

Easy to reproduce the bug:

$ PERL5LIB=/tmp eb CUDA-9.2.88.eb
...
== installing...
== ... (took < 1 sec)
ERROR: Traceback (most recent call last):
  File "/lib/python2.7/site-packages/easybuild_framework-4.4.2.dev0-py2.7.egg/easybuild/main.py", line 118, in build_and_install_software
    (ec_res['success'], app_log, err) = build_and_install_one(ec, init_env)
  File "/lib/python2.7/site-packages/easybuild_framework-4.4.2.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 3713, in build_and_install_one
    result = app.run_all_steps(run_test_cases=run_test_cases)
  File "/lib/python2.7/site-packages/easybuild_framework-4.4.2.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 3601, in run_all_steps
    self.run_step(step_name, step_methods)
  File "/lib/python2.7/site-packages/easybuild_framework-4.4.2.dev0-py2.7.egg/easybuild/framework/easyblock.py", line 3454, in run_step
    step_method(self)()
  File "/lib/python2.7/site-packages/easybuild_easyblocks-4.4.2.dev0-py2.7.egg/easybuild/easyblocks/c/cuda.py", line 176, in install_step
    p5lib = os.pathsep.join(self.builddir, p5lib)
TypeError: join() takes exactly one argument (2 given)

Was introduced in #2387 (so present in EasyBuild 4.4.0 and 4.4.1)