enhance IntelBase easyblock to avoid crash when $USER is not set

[branfosj]

(created using eb --new-pr)

This sets a default for the case where USER is not set. The alternative would be to check that USER is set and give a better error if it is not.

Error, from slack:

== 2021-12-20 01:28:39,758 build_log.py:169 ERROR EasyBuild crashed with an error (at easybuild/base/exceptions.py:124 in __init__): Traceback (most recent call last):
  File "/store/software/EasyBuild/4.4.2/lib/python2.7/site-packages/easybuild/main.py", line 118, in build_and_install_software
    (ec_res['success'], app_log, err) = build_and_install_one(ec, init_env)
  File "/store/software/EasyBuild/4.4.2/lib/python2.7/site-packages/easybuild/framework/easyblock.py", line 3676, in build_and_install_one
    app = app_class(ecdict['ec'])
  File "/store/software/EasyBuild/4.4.2/lib/python2.7/site-packages/easybuild/easyblocks/i/intel_compilers.py", line 48, in __init__
    super(EB_intel_minus_compilers, self).__init__(*args, **kwargs)
  File "/store/software/EasyBuild/4.4.2/lib/python2.7/site-packages/easybuild/easyblocks/generic/intelbase.py", line 110, in __init__
    self.home_subdir_local = os.path.join(common_tmp_dir, os.getenv('USER'), 'easybuild_intel')
  File "/usr/lib64/python2.7/posixpath.py", line 75, in join
    if b.startswith('/'):
AttributeError: 'NoneType' object has no attribute 'startswith'
 (at easybuild/main.py:151 in build_and_install_software)

[boegel]

lgtm

[boegel]

Test report by @boegel

Overview of tested easyconfigs (in order)

• SUCCESS intel-compilers-2021.4.0.eb
• SUCCESS impi-2021.4.0-intel-compilers-2021.4.0.eb

Build succeeded for 2 out of 2 (2 easyconfigs in total)
node3104.skitty.os - Linux centos linux 7.9.2009, x86_64, Intel(R) Xeon(R) Gold 6140 CPU @ 2.30GHz (skylake_avx512), Python 3.6.8
See https://gist.github.com/f914b2358f0142d0b3f3ccb8ebf20895 for a full test report.