Remove QPY generation venvs more eagerly

Previously, the venv was only tidied up at the end of the script.  We
now have enough published versions of Qiskit that if a script fails its
QPY runs (and so exited before cleaning up its venv), we could overflow
the disk space allocated to the VM, and get no output from the job.

test/qpy_compat/process_version.sh

@@ -52,12 +52,13 @@ if [[ ! -d $cache_dir ]] ; then
     mkdir -p "$cache_dir"
     pushd "$cache_dir"
     echo "Generating QPY files with $package==$version"
-    "$venv_dir/bin/python" "${our_dir}/test_qpy.py" generate --version="$version"
+    # If the generation script fails, we still want to tidy up before exiting.
+    "$venv_dir/bin/python" "${our_dir}/test_qpy.py" generate --version="$version" || { rm -rf "$venv_dir"; exit 1; }
+    rm -rf "$venv_dir"
 else
     echo "Using cached QPY files for $version"
     pushd "${cache_dir}"
 fi
 echo "Loading qpy files from $version with dev Qiskit"
 "$python" "${our_dir}/test_qpy.py" load --version="$version"
 popd
-rm -rf "$venv_dir"