ci: name the interop interpreter instead of relying on $GITHUB_PATH

The workflow already installed h5py into /opt/interop and set
CLAWHDF5_REQUIRE_INTEROP=1, but it reached the tests only by appending
that venv to $GITHUB_PATH, which Gitea's runner does not reliably
propagate into test subprocesses. If `python3` resolved to the system
interpreter instead, every interop suite would skip. Setting
CLAWHDF5_PYTHON outright removes the question: together with
REQUIRE_INTEROP the suites either run or the build goes red.

Verified both directions locally — with a venv the four suites run 94
tests green; with a bogus interpreter and REQUIRE_INTEROP=1 the facade
and netCDF4 suites fail 22 tests rather than skipping.

Also documents creating the local `.venv` that `ci-test.sh` detects.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

.gitea/workflows/ci.yml

@@ -33,8 +33,14 @@ jobs:
           /opt/interop/bin/pip install --no-cache-dir h5py numpy netCDF4 xarray
           echo "/opt/interop/bin" >> "$GITHUB_PATH"
       - name: Show interop library versions
-        run: python3 -c "import h5py, netCDF4; print('h5py', h5py.__version__, 'HDF5', h5py.version.hdf5_version, 'netCDF4', netCDF4.__version__)"
+        run: /opt/interop/bin/python -c "import h5py, netCDF4; print('h5py', h5py.__version__, 'HDF5', h5py.version.hdf5_version, 'netCDF4', netCDF4.__version__)"
       - name: Run CI script
         env:
+          # Name the interpreter outright rather than relying on $GITHUB_PATH
+          # reaching the test processes: if `python3` resolved to the system
+          # one instead of the venv, every interop suite would skip.
+          # CLAWHDF5_REQUIRE_INTEROP turns that skip into a failure, so the
+          # two together mean the suites either run or the build goes red.
+          CLAWHDF5_PYTHON: /opt/interop/bin/python
           CLAWHDF5_REQUIRE_INTEROP: "1"
         run: bash scripts/ci-test.sh