Pin Rust toolchain action to specific commit SHA for security

Use dtolnay/rust-toolchain@efa25f7
instead of @stable to follow GitHub Actions security best practices
of pinning third-party actions to immutable commit SHAs.

Author: aclark4life

.github/workflows/test-python.yml

@@ -83,7 +83,7 @@ jobs:
           python-version: ${{ matrix.python-version }}
       - name: Install Rust toolchain
         if: matrix.extension == 'rust'
-        uses: dtolnay/rust-toolchain@stable
+        uses: dtolnay/rust-toolchain@efa25f7f19611383d5b0ccf2d1c8914531636bf9 # stable
       - id: setup-mongodb
         uses: mongodb-labs/drivers-evergreen-tools@master
         with: