docker-run-tests: pass through PYTHON env var

problem: we can't pass an explicit python executable path through from
the matrix

solution: add it to the list

Author: trws

src/test/docker/docker-run-checks.sh

@@ -231,6 +231,7 @@ else
         -e TAP_DRIVER_QUIET \
         -e FLUX_TEST_TIMEOUT \
         -e FLUX_TEST_SIZE_MAX \
+        -e PYTHON \
         -e PYTHON_VERSION \
         -e PRELOAD \
         -e POISON \