Drop environment proxy resolver to lowest priority

Currently the environment proxy resolver has 100 priority. The portal
resolver in GLib has 90 priority, the GNOME settings resolver has 80
priority, and the libproxy resolver is relegated to last place with 0
priority. Making the new environment proxy resolver highest-priority
seemed like a good idea to me because environment variables are for
debugging and surely if an environment variable is set it's because you
want it to be respected, right?

Wrong. Problem is, Console and Terminal both inspect GNOME proxy
settings and translate them into environment variables, see console#81.
If we prioritize the environment variables, which are unable to fully
express the desktop configuration, then we lose out on goodies like
web proxy autoconfig and such. Worse, this will only happen when running
from your terminal, meaning applications will break only when run in
Console or Terminal. Imagine how frustrating that would be to debug.

Although I'm usually reluctant to make major behavior changes in a
stable release, we should revert back to longstanding historical
behavior. The environment proxy resolver will be used only as the last
fallback, useful in case libproxy is not available.

Fixes #190

Part-of: <https://gitlab.gnome.org/GNOME/glib-networking/-/merge_requests/217>


(cherry picked from commit 6e2cf4fb)