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.

CLI/go-build-generic

@@ -0,0 +1,35 @@
+#!/bin/bash
+
+set -euo pipefail
+
+if [[ $# -ne 1 ]]; then
+	echo "" 1>&2
+	echo "Compiler for a Go PACKAGE producing GOOS/GOARCH binaries in the" 1>&2
+	echo "top-level directory (should not be used for releasing)." 1>&2
+	echo "" 1>&2
+	echo "usage: $0 PACKAGE..." 1>&2
+	echo "" 1>&2
+	echo "Features:" 1>&2
+	echo "" 1>&2
+	echo "* automatically sets -tags=ooni_psiphon_config when possible;" 1>&2
+	echo "" 1>&2
+	echo "* if GOLANG_EXTRA_FLAGS is set, pass it to the Go compiler." 1>&2
+	echo "" 1>&2
+	echo "Example:" 1>&2
+	echo "" 1>&2
+	echo "  ./CLI/go-build-generic ./internal/cmd/miniooni" 1>&2
+	echo "" 1>&2
+	exit 1
+fi
+
+if [[ -f ./internal/engine/psiphon-config.json.age &&
+	-f ./internal/engine/psiphon-config.key ]]; then
+	OONI_PSIPHON_TAGS=ooni_psiphon_config
+else
+	OONI_PSIPHON_TAGS=""
+fi
+
+set -x
+for pkg in "$@"; do
+	go build -tags=$OONI_PSIPHON_TAGS -ldflags="-s -w" ${GOLANG_EXTRA_FLAGS:-} $pkg
+done