mdtraj: Install pyarrow from wheels only in tests

mdtraj[all] pulls in pyarrow unpinned; pyarrow 26.0.0 has no riscv64
wheel on pypi.riseproject.dev yet, so pip built it from sdist and CMake
failed to find Arrow. Add pyarrow to PIP_ONLY_BINARY so pip falls back to
the newest version with a riscv64 wheel (25.0.1 today).

Author: luhenry

.github/workflows/build-mdtraj.yml

@@ -84,7 +84,7 @@ jobs:
           CIBW_MANYLINUX_RISCV64_IMAGE: ${{ env.MANYLINUX_RISCV64_IMAGE }}
           CIBW_ENVIRONMENT: >-
             PIP_EXTRA_INDEX_URL=https://pypi.riseproject.dev/simple/
-            PIP_ONLY_BINARY=numpy,scipy,pandas,tables,blosc2,numexpr,ndindex,msgpack,pydantic-core,netcdf4,cftime
+            PIP_ONLY_BINARY=numpy,scipy,pandas,pyarrow,tables,blosc2,numexpr,ndindex,msgpack,pydantic-core,netcdf4,cftime
           # Upstream's wheel job only smoke-tests; also run its CI suite against the wheel,
           # with its conda test env minus openmm (no riscv64 build; those tests skip).
           CIBW_TEST_EXTRAS: all