sed expression in tools/tox-venv is not POSIX compatible

[Mina Galić]

When running tools/tox-venv py3 on FreeBSD, I get the following Error:

sed: 2: ":x; /\\$/ { N; s/\\\n[ ...": unused label 'x; /\\$/ { N; s/\\\n[ ]*//; tx };'

This suggests to me, that the expression

        sed -e ':x; /\\$/ { N; s/\\\n[ ]*//; tx };' "${tox_ini}" |

is not POSIX compatible.

Looking at GNU sed's documentation, I see the first issue with making this expression POSIX compliant will be that `N` behaves intentionally different on GNU sed than on POSIX:

https://www.gnu.org/software/sed/manual/html_node/Reporting-Bugs.html#Reporting-Bugs

So the solution here might be simply to require GNU sed, and put it's path/name into an environment variable that can be overriden from the outside.

[Dan Watkins (oddbloke)]

Triaging this as Low, as there's generally a way to do whatever tox-venv does without using that script.

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3501