chore: get rid of now-stabilized error_in_core feature

[TheButlah]

(continuation of #258 )

Attempting to build the SDK without stable compiler features is not possible right now due to the use of #![feature(error_in_core)]. However, this feature was stabilized in rust 1.81 in September 2024.

It would be good to remove the now-unneeded unstable flag.

[ivila]

pipeline fails with

error: an inner attribute is not permitted in this context
 --> /__w/teaclave-trustzone-sdk/teaclave-trustzone-sdk/optee-utee/target/debug/build/optee-utee-d980c343d6a5328f/out/error_in_core.rs:1:1
  |
1 | #![feature = "error_in_core"]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: inner attributes, like `#![no_std]`, annotate the item enclosing them, and are usually found at the beginning of source files
  = note: outer attributes, like `#[test]`, annotate the item following them

And please add a valid license header to optee-utee/build.rs

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you 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.

[ivila]

Should be: format!("#![feature({})]\n", $feat)

[ivila]

Please add a valid license header.

[TheButlah]

Unfortunately it looks like the code doesn't work how I thought it would :(

[TheButlah]

Closing. I don't think there's a good solution here other than upgrading the stdlib fork and afterwards the rust-toolchain.toml.

[ivila]

Fixed by #262