build: pin the Rust toolchain to 1.94.1

Release engines are built on four hosts plus a container that installs
its own Rust. Unpinned, they ran three different rustc versions. A
rust-toolchain.toml makes rustup on every host, and in the linux-arm64
container, use the same compiler.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017rVbt7rENTwXkdHt3Bpgb5

Author: anvanster

rust-toolchain.toml

@@ -0,0 +1,8 @@
+# One compiler for every release build. Each platform's engine is built on a
+# different host (and linux-arm64 in a container that installs Rust itself);
+# without a pin they drifted onto three different rustc versions, so one release
+# shipped binaries the same source compiled differently.
+[toolchain]
+channel = "1.94.1"
+components = ["rustfmt", "clippy"]
+profile = "minimal"