Add build test and update issue template

[cderici]

Description

This does two things:

1. Adds a Makefile target called build-test that runs the setup.py sdist and installs the artifact in a virtual environment and runs a very simple command on it. The purpose of this is to catch the build and runtime dependency problems early on whenever a change is made that'll effect the release down the road (i.e. before it blows up when a new version is released). We add it into a GH action job as well to automatically catch these issues.
   For example, having something like this would've saved us from hitting Libjuju 3.3.1.0 is unusable #1025 on the 3.3.1.0 release, which is the sole reason we had to make a new release https://github.com/juju/python-libjuju/releases/tag/3.3.1.1 afterwards.

2. Updates the bug report template and adds Please provide a simplified reproducer, and if it's possible please refrain from providing a "clone this repository and run the integration tests to see the problem" type of a reproducer. Thanks!, which is pretty self-explanatory.

QA Steps

Just running the make target to see if it succeeds locally should be sufficient to QA this:

make build-test

And maybe some manual checking for typos in the template message (b/c user-facing) could help.

[cderici]

/merge