Use a fixed nightly version for the wasm toolchain

.github/actions/build-dependencies/action.yml

@@ -28,9 +28,13 @@ runs:
         toolchain: ${{ inputs.rust-toolchain }}
         components: ${{ inputs.rust-components }}
 
+    - name: Get nightly version to use
+      id: nightly
+      run: echo "::set-output name=version::$(cat .github/nightly-version)"
+
     - name: Install WASM toolchain
       uses: actions-rs/toolchain@v1
       with:
-        toolchain: nightly
+        toolchain: ${{ steps.nightly.outputs.version }}
         profile: minimal
         target: wasm32-unknown-unknown