Fix Lua 5.5 support

Author: khvzak

src/luarocks/build/rust-mlua.lua

@@ -74,9 +74,9 @@ function mlua.run(rockspec, no_install)
         local libdir = path.lib_dir(rockspec.name, rockspec.version)
 
         fs.make_dir(dir.dir_name(libdir))
-        for mod, rustlib_name in pairs(rockspec.build.modules) do
+        for k, rustlib_name in pairs(rockspec.build.modules) do
             -- Make loop variable mutable in Lua 5.5
-            local mod
+            local mod = k
 
             -- If `mod` is a number, then it's an array entry
             if type(mod) == "number" then