Add missing gsub local binding

Needed after commit 6a9f282 (Move runcmd() to l3build-aux, 2023-07-17).

l3build-aux.lua

@@ -25,6 +25,7 @@ for those people who are interested.
 -- local safety guards and shortcuts
 
 local match = string.match
+local gsub  = string.gsub
 
 local pairs = pairs
 local print = print