Hardcode rustc to 1.60.0 for Python CI (#69)

rustc >=1.61.0 breaks the build of our Python API (see https://github.com/rust-lang/rust/blob/1.61.0/RELEASES.md#compatibility-notes).

Lets wait for rust-lang/cc-rs#671 to land and hardcode the CI environment to 1.60.0 for now.

Author: felixwilhelm

.github/workflows/python.yml

@@ -12,8 +12,7 @@ jobs:
       - name: Install latest rust toolchain
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: stable
-          default: true
+          toolchain: 1.60.0
           override: true
 
       - name: install python dependencies