fix: set NVM_HAS_RUN (#60)

Author: sxzz

functions/nvm.fish

@@ -19,6 +19,6 @@ function nvm
   end
 
   if test $argv[1] = "use"; or test $argv[1] = "install"
-    set -g NVM_HAS_RUN 1
+    set -gx NVM_HAS_RUN 1
   end
 end