Add rust-toolchain.toml (#959)

When I was doing some testing I noticed that if I have a different rust
version installed and attempt to rebuild the `stable-mir-json` artefacts
that they were error on changed MIR data structures. Adding a
`rust-toolchain.toml` means that the correct version will always be
active for the repo.

```
daniel@daniel-MS-7E06 mir-semantics$ rustup show active-toolchain -v
nightly-2024-11-29-x86_64-unknown-linux-gnu
active because: overridden by '/home/daniel/Applications/mir-semantics/rust-toolchain.toml'
```

I also updated the github workflow so that it if `stable-mir-json` is
updated it will update the channel to the new one.

---------

Co-authored-by: automergerpr-permission-manager[bot] <190534181+automergerpr-permission-manager[bot]@users.noreply.github.com>

Author: dkcumming

.github/workflows/update-dependencies.yml

@@ -50,7 +50,9 @@ jobs:
           git checkout "${VERSION}"
           cd ../..
           sed -i 's!__smir_version__: Final = '"'[0-9a-f]*'"'!__smir_version__: Final = '"'${VERSION}'"'!' kmir/src/kmir/__init__.py
-          git add deps/stable-mir-json kmir/src/kmir/__init__.py
+          CHANNEL=$(grep '^channel' deps/stable-mir-json/rust-toolchain.toml | sed 's!channel = "\(.*\)"!\1!')
+          sed -i 's!^channel = ".*"!channel = "'"${CHANNEL}"'"!' rust-toolchain.toml
+          git add deps/stable-mir-json kmir/src/kmir/__init__.py rust-toolchain.toml
           git commit -m "deps/stable-mir-json: sync submodule ${VERSION}" || true
       - name: 'Update Nix flake inputs'
         run: |

rust-toolchain.toml

@@ -0,0 +1,3 @@
+[toolchain]
+channel = "nightly-2024-11-29"
+components = ["llvm-tools", "rustc-dev", "rust-src", "rust-analyzer"]