patch 8.2.0391: CI test coverage dropped

Problem:    CI test coverage dropped.
Solution:   Set $DISPLAY also for non-GUI builds. (James McCoy, closes #5788)
diff --git a/src/version.c b/src/version.c
index 33bdb39..78d8940 100644
--- a/src/version.c
+++ b/src/version.c
@@ -739,6 +739,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    391,
+/**/
     390,
 /**/
     389,