Replace release.mk by release scripts

Since commit 2687d20280, the Makefile
named release.mk stopped handling dependencies between recipes, because
they have to be executed separately (from different Github Actions
jobs).

There is no real benefit using a Makefile anymore. Replace them by
several individual release scripts for simplicity and readability.

release/test_client.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -ex
+cd "$(dirname ${BASH_SOURCE[0]})"
+. build_common
+cd .. # root project dir
+
+TEST_BUILD_DIR="$WORK_DIR/build-test"
+
+rm -rf "$TEST_BUILD_DIR"
+meson setup "$TEST_BUILD_DIR" -Dcompile_server=false \
+    -Db_sanitize=address,undefined
+ninja -C "$TEST_BUILD_DIR" test