pio-test: global library extra_script.py is executed with an empty project

Notes

Fixing this issue breaks "mbed" framework. See https://github.com/platformio/platformio-core/runs/5156160446?check_suite_focus=true

Configuration

Operating system:
Fedora 33

PlatformIO Version (platformio --version):

PlatformIO Core             5.2.0a3
Python                      3.9.1-final.0
System Type                 linux_x86_64

Description of problem

Globally installed library extra_script.py is executed when running pio-test:

> pio test -e test
Verbose mode can be enabled via `-v, --verbose` option
Collected 1 items

Processing * in test environment
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Building...
TypeError: join() argument must be str, bytes, or os.PathLike object, not 'NoneType':
  File "/home/maxim/.platformio/penv/lib/python3.9/site-packages/platformio/builder/main.py", line 178:
    env.SConscript("$BUILD_SCRIPT")
  File "/home/maxim/.platformio/packages/tool-scons/scons-local-4.1.0/SCons/Script/SConscript.py", line 591:
    return _SConscript(self.fs, *files, **subst_kw)
  File "/home/maxim/.platformio/packages/tool-scons/scons-local-4.1.0/SCons/Script/SConscript.py", line 280:
    exec(compile(scriptdata, scriptname, 'exec'), call_stack[-1].globals)
  File "/home/maxim/.platformio/platforms/native/builder/main.py", line 43:
    target_bin = env.BuildProgram()
  File "/home/maxim/.platformio/packages/tool-scons/scons-local-4.1.0/SCons/Util.py", line 658:
    return self.method(*nargs, **kwargs)
  File "/home/maxim/.platformio/penv/lib/python3.9/site-packages/platformio/builder/tools/platformio.py", line 62:
    env.ProcessProjectDeps()
  File "/home/maxim/.platformio/packages/tool-scons/scons-local-4.1.0/SCons/Util.py", line 658:
    return self.method(*nargs, **kwargs)
  File "/home/maxim/.platformio/penv/lib/python3.9/site-packages/platformio/builder/tools/platformio.py", line 140:
    project_lib_builder = env.ConfigureProjectLibBuilder()
  File "/home/maxim/.platformio/packages/tool-scons/scons-local-4.1.0/SCons/Util.py", line 658:
    return self.method(*nargs, **kwargs)
  File "/home/maxim/.platformio/penv/lib/python3.9/site-packages/platformio/builder/tools/piolib.py", line 1064:
    lib_builders = env.GetLibBuilders()
  File "/home/maxim/.platformio/packages/tool-scons/scons-local-4.1.0/SCons/Util.py", line 658:
    return self.method(*nargs, **kwargs)
  File "/home/maxim/.platformio/penv/lib/python3.9/site-packages/platformio/builder/tools/piolib.py", line 993:
    lb = LibBuilderFactory.new(env, lib_dir)
  File "/home/maxim/.platformio/penv/lib/python3.9/site-packages/platformio/builder/tools/piolib.py", line 61:
    obj = getattr(sys.modules[__name__], clsname)(env, path, verbose=verbose)
  File "/home/maxim/.platformio/penv/lib/python3.9/site-packages/platformio/builder/tools/piolib.py", line 137:
    self.process_extra_options()
  File "/home/maxim/.platformio/penv/lib/python3.9/site-packages/platformio/builder/tools/piolib.py", line 280:
    self.env.SConscript(
  File "/home/maxim/.platformio/packages/tool-scons/scons-local-4.1.0/SCons/Script/SConscript.py", line 591:
    return _SConscript(self.fs, *files, **subst_kw)
  File "/home/maxim/.platformio/packages/tool-scons/scons-local-4.1.0/SCons/Script/SConscript.py", line 280:
    exec(compile(scriptdata, scriptname, 'exec'), call_stack[-1].globals)
  File "/home/maxim/.platformio/lib/BSEC Software Library/extra_script.py", line 5:
    LIBPATH=[realpath(join('src', env.get('BOARD_MCU')))],
  File "/usr/lib64/python3.9/posixpath.py", line 90:
    genericpath._check_arg_types('join', a, *p)
  File "/usr/lib64/python3.9/genericpath.py", line 152:
    raise TypeError(f'{funcname}() argument must be str, bytes, or '
======================================================================== [FAILED] Took 0.33 seconds ========================================================================

Test    Environment    Status    Duration
------  -------------  --------  ------------
*       test           FAILED    00:00:00.329
================================================================== 1 failed, 0 succeeded in 00:00:00.329 ==================================================================

Steps to Reproduce

1. Make sure ~/.platformio/lib is empty before the test
2. pio lib -g install "https://github.com/BoschSensortec/BSEC-Arduino-library.git#v1.6.1480"
3. Create a dummy project project in some directory
4. pio test -e test triggers an exception (see above)

If problems with PlatformIO Build System:

The content of platformio.ini:

[env:test]
platform = native

Additional info

As seen in the extra_script of the lib
https://github.com/BoschSensortec/BSEC-Arduino-library/blob/7a9357566c75048331c15b55a4eb20f1de14f36e/extra_script.py

Import('env')
from os.path import join, realpath

env.Append(
    LIBPATH=[realpath(join('src', env.get('BOARD_MCU')))],
    LIBS=['algobsec']
)

As seen in the trace:

File "/home/maxim/.platformio/lib/BSEC Software Library/extra_script.py", line 5:
    LIBPATH=[realpath(join('src', env.get('BOARD_MCU')))],

It tries to use BOARD_MCU, which is not available with the platform. But, is it an expected behaviour with the global libraries that scripts are executed unconditionally?