update-deps: fix runtime error [ci skip]

* tool/update-deps (init_global): environment strings are frozen.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65791 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/update-deps

@@ -239,7 +239,7 @@ DEPENDENCIES_SECTION_END_MARK = "\# AUTOGENERATED DEPENDENCIES END\n"
 
 def init_global
   ENV['LC_ALL'] = 'C'
-  if mkflag = ENV['GNUMAKEFLAGS'] and mkflag.sub!(/(\A|\s+)-j\d*(?=\s+|\z)/, '')
+  if mkflag0 = ENV['GNUMAKEFLAGS'] and (mkflag = mkflag0.sub(/(\A|\s+)-j\d*(?=\s+|\z)/, '')) != mkflag0
     mkflag.strip!
     ENV['GNUMAKEFLAGS'] = mkflag
   end