pretty_tox.sh: Portablity improvement

A recent change (commit 0d5a11d9c722870f9c5e31a993219c7e240b4e19)
introduced bash dependency.  It broke my environment, where bash
is not available.  This change aims to restore it.

As far as I undestand, the change in question requires bash for
the "pipefail" feature.  Fortunately there's a more portable
well-known trick for the purpose.

Closes-Bug: #1398266
Change-Id: Id3561d81462923d20ad39584ba4cf8e1a294a8a8
Merge "pretty_tox.sh: Portablity improvement"