ci: add stable build, add tests.

.github/ci/build-stable.sh

@@ -0,0 +1,16 @@
+#!/bin/bash
+## on push branch~=gh-readonly-queue/main/.*
+## on pull_request
+
+set -euo pipefail
+
+export RUSTUP_HOME=/ci/cache/rustup
+export CARGO_HOME=/ci/cache/cargo
+export CARGO_TARGET_DIR=/ci/cache/target
+
+hashtime restore /ci/cache/filetime.json || true
+hashtime save /ci/cache/filetime.json
+
+sed -i 's/channel.*/channel = "stable"/g' rust-toolchain.toml
+
+./ci_stable.sh