d/rules: avoid overwriting PYBUILD_BEFORE_TEST when running on jenkins

Gbp-Dch: Ignore
Signed-off-by: Mattia Rizzolo <mattia@debian.org>

debian/rules

@@ -27,7 +27,7 @@ export PYBUILD_AFTER_TEST = debian/tests/basic-command-line
 export PYBUILD_AFTER_INSTALL = rm -rvf '{destdir}/{install_dir}/htmlcov' '{destdir}/{install_dir}/.coverage'
 
 ifneq ($(JENKINS_EXPORTS_DIR),)
-  export PYBUILD_BEFORE_TEST = mkdir -pv $(JENKINS_EXPORTS_DIR)/test-results
+  export PYBUILD_BEFORE_TEST += mkdir -pv $(JENKINS_EXPORTS_DIR)/test-results
   export PYBUILD_TEST_ARGS += --junit-xml=$(JENKINS_EXPORTS_DIR)/test-results/tests.xml
 endif