Add explicit toolchain input for pinned Rust toolchain action

When using a pinned commit SHA instead of a tag, the action requires
an explicit 'toolchain' input to specify which Rust version to install.

Author: aclark4life

.github/workflows/test-python.yml

@@ -84,6 +84,8 @@ jobs:
       - name: Install Rust toolchain
         if: matrix.extension == 'rust'
         uses: dtolnay/rust-toolchain@efa25f7f19611383d5b0ccf2d1c8914531636bf9 # stable
+        with:
+          toolchain: stable
       - id: setup-mongodb
         uses: mongodb-labs/drivers-evergreen-tools@master
         with: