Revert "update toolchain in rust action"

This reverts commit b20ea8d.

Author: trappitsch

.github/workflows/rust.yml

@@ -96,7 +96,7 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: 1.81.0
+          toolchain: 1.72.0
           target: wasm32-unknown-unknown
           override: true
       - name: Download and install Trunk binary