tox.ini: Prevent casual addition of bash dependency

While bash is not available on every environments, there seems to be
a tendency to introduce more bash dependency very casually.  This check
is intended to be a reminder to give people a chance to consider
alternatives.

Related-Bug: #1398266
Change-Id: I5103e0d7c111880d9a02dd93de56c567066ed14c
Merge "tox.ini: Prevent casual addition of bash dependency"