package/lttng-libust: properly append to LTTNG_LIBUST_CONF_ENV in condition

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/lttng-libust/lttng-libust.mk

@@ -27,7 +27,7 @@ else ifeq ($(BR2_PACKAGE_PYTHON3),y)
 LTTNG_LIBUST_DEPENDENCIES += python3
 LTTNG_LIBUST_CONF_OPTS += --enable-python-agent
 else
-LTTNG_LIBUST_CONF_ENV = am_cv_pathless_PYTHON="none"
+LTTNG_LIBUST_CONF_ENV += am_cv_pathless_PYTHON="none"
 LTTNG_LIBUST_CONF_OPTS += --disable-python-agent
 endif