cli: add support for passing proxy command line parameter (#981)

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.

cmd/ooniprobe/internal/ooni/ooni_test.go

@@ -17,7 +17,7 @@ func TestInit(t *testing.T) {
 	probe := NewProbe("", ooniHome)
 	swName := "ooniprobe-cli-tests"
 	swVersion := "3.0.0-alpha"
-	if err := probe.Init(swName, swVersion); err != nil {
+	if err := probe.Init(swName, swVersion, ""); err != nil {
 		t.Error(err)
 		t.Fatal("failed to init the context")
 	}