CI - Use rust-toolchain-file everywhere (#3872)

# Description of Changes

We were using `rust-toolchain` in some places, and `rust-toolchain-file`
in others. I think Rust released a new version, which made the
`rust-toolchain` parts break with:

```
info: downloading component 'rustfmt'
info: removing previous version of component 'rust-src'
info: rolling back changes
error: could not rename component file from '/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test/src/term/terminfo/searcher' to '/root/.rustup/tmp/a4eo07uz83vsyfhk_dir/bk': Invalid cross-device link (os error 18)
Error: Process completed with exit code 1.
```

(Separately, this breakage is confusing.. we'll probably run into this
again when we roll forward our rust version..)

# API and ABI breaking changes

None. CI-only change.

# Expected complexity level and risk

1

# Testing

- [x] CI passes
- [x] There are no more instances of `rust-toolchain`:
```
$ grep -rIP 'rust-toolchain(?!-file)' .github/workflows
.github/CODEOWNERS:/rust-toolchain.toml @cloutiertyler
```
(on `master`, this finds the instances we changed)

---------

Co-authored-by: Zeke Foppa <bfops@users.noreply.github.com>

Author: bfops

.github/workflows/ci.yml

@@ -550,7 +550,7 @@ jobs:
           cat Cargo.toml
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dsherret/rust-toolchain-file@v1
 
       - name: Cache Rust dependencies
         uses: Swatinem/rust-cache@v2
@@ -674,7 +674,7 @@ jobs:
         run: dotnet format --no-restore --verify-no-changes SpacetimeDB.ClientSDK.sln
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dsherret/rust-toolchain-file@v1
 
       - name: Cache Rust dependencies
         uses: Swatinem/rust-cache@v2

.github/workflows/typescript-test.yml

@@ -81,7 +81,7 @@ jobs:
       #     echo "Using SpacetimeDB branch from file: $branch"
 
       - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
+        uses: dsherret/rust-toolchain-file@v1
 
       - name: Cache Rust dependencies
         uses: Swatinem/rust-cache@v2