[TST]: enable Rust backtraces in Python binding tests (#4253)

## Description of changes

Hoping this helps debug test failures like this:
https://github.com/chroma-core/chroma/actions/runs/14366566402/job/40280832204.

Author: codetheweb

.github/workflows/_python-tests.yml

@@ -61,6 +61,7 @@ jobs:
         env:
           PROPERTY_TESTING_PRESET: ${{ inputs.property_testing_preset }}
           CHROMA_RUST_BINDINGS_TEST_ONLY: "1"
+          RUST_BACKTRACE: 1
 
   test-rust-single-node-integration:
     strategy: