patch 8.2.2507: github build may fail if Ubuntu 20.04 is used

Problem:    Github build may fail if Ubuntu 20.04 is used.  Installing rust is
            not needed.
Solution:   Specify ubuntu-18.04 instead of latest. Update "pip" instead of
            installing rust. (Ozaki Kiichi, closes #7820)
diff --git a/src/version.c b/src/version.c
index 6d01fa8..eeddd86 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2507,
+/**/
     2506,
 /**/
     2505,