Install meta tester at system level again

[HebaruSan]

Problem

The meta tester is broken.

Cause

In #3958 I added --user to the pip3 install, because I thought it would be cleaner than copying /usr/local from one image to the next. However, when it runs these images, GitHub sets $HOME to /github/home, which they map to a folder they control:

This takes /root/.local/lib/python3.10, which is where all our modules are installed, out of Python's module search path, which breaks everything. (It also makes for a very confusing problem because my docker run commands seemed to work fine until I spotted HOME in that long list of environment variables.)

Changes

Now we install to /usr/local again and copy it forward. Impact on image size is negligible:

REPOSITORY                         TAG       IMAGE ID       CREATED             SIZE
meta_tester_testing                latest    7e633c3513db   11 minutes ago      376MB