chore(rustup): add rust-toolchain.toml config

Specify the latest compatible toolchain in the rust-toolchain.toml file.
This will give better stability in local development and CI, without
random breakages and new warning to solve.

Currently we use the beta channel for an issue in rust-analyzer on
1.85.1, where diagnostics are cleared. But we should move to the stable
channel on next release.

Signed-off-by: Joshua Chapman <joshua.chapman@secomind.com>

Author: joshuachp

rust-toolchain.toml

@@ -0,0 +1,22 @@
+# This file is part of Astarte.
+#
+# Copyright 2025 SECO Mind Srl
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+
+[toolchain]
+channel = "1.87.0-beta.1"
+components = ["rust-analyzer", "llvm-tools"]
+profile = "default"