This implements the bare minimum needed to have circumvention proxy support in OONI Probe CLI.
The reference issue is https://github.com/ooni/probe/issues/1955. (Most of that issue is implemented, save for the fact that currently we do not have support for `http` and `https` proxies.)
While there, add to `Makefile` a rule for correctly building `ooniprobe` and `miniooni` for "this system" (i.e., the default `GOOS` and `GOARCH` on a system), because we needed this for testing this patch and we needed to figure out the commands instead.
