Pin GitHub CI Rust toolchain to 1.93.0

Using the floating stable toolchain currently breaks this project. Repro via just build-debug:

```
Compiling matrix-sdk v0.16.0
error: queries overflow the depth limit!
help: consider increasing the recursion limit by adding #![recursion_limit = "256"] to your crate (matrix_sdk)
note: query depth increased by 130 when computing layout of matrix-sdk Client::sync async body
error: could not compile matrix-sdk (lib) due to 1 previous error
```

Pinning CI to 1.93.0 keeps CI on the known-good toolchain until upstream/toolchain compatibility is addressed.
We should have had that to begin with (we're pinning as much as we can anyway), but..

Ref: https://github.com/etkecc/baibot/pull/83#issuecomment-4008463792

.github/workflows/workflow.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6
-      - uses: dtolnay/rust-toolchain@stable
+      - uses: dtolnay/rust-toolchain@1.93.0
       - name: Install SQLite3
         run: sudo apt-get update && sudo apt-get install -y libsqlite3-dev
       - run: cargo test --all-features