fs_violations for ports that build using python tools

[Adriaan de Groot]

I build many ports in poudriere with the `-t` (test port) flag. Ports that use 
Python-based tools tend to fail with a fs_violation. For instance, FreeCAD:

=>> Checking for filesystem violations... done
=>> Error: Filesystem touched during build:
extra: usr/local/lib/python3.7/site-packages/shiboken2/__pycache__
extra: usr/local/lib/python3.7/site-packages/shiboken2/files.dir/
shibokensupport/__pycache__
extra: usr/local/lib/python3.7/site-packages/shiboken2/files.dir/
shibokensupport/signature/lib/__pycache__

or nextpnr:

=>> Checking for filesystem violations... done
=>> Error: Filesystem touched during build:
extra: usr/local/share/trellis/util/common/__pycache__
extra: usr/local/share/trellis/timing/util/__pycache__

On the build cluster, these don't seem to cause problems. Note that these 
pycache files are **not** from the port being built, but from other, python-
based, tools that are run as part of the build: trellis is a BUILD_DEPENDS for 
nextpnr.

What can I do to resolve this kind of build failure? I'd like to avoid false-
failures because they mask and confuse local exp-runs for (say) CMake updates.

[ade]
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