Fix `fnm env` for fish shell. (#91)

I added `-gx` to the `set` commands for fish so that the variables can be picked up by later runs of fnm. There are cases where the setup current code already works however it doesn't when the `FNM_...` variables haven't been exported globally before.

executable/Env.re

@@ -104,12 +104,12 @@ let run = (~forceShell, ~multishell, ~nodeDistMirror, ~fnmDir, ~useOnCd) => {
     )
     |> Console.log;
   | Fish =>
-    Printf.sprintf("set PATH %s/bin $PATH;", path) |> Console.log;
-    Printf.sprintf("set %s %s;", Config.FNM_MULTISHELL_PATH.name, path)
+    Printf.sprintf("set -gx PATH %s/bin $PATH;", path) |> Console.log;
+    Printf.sprintf("set -gx %s %s;", Config.FNM_MULTISHELL_PATH.name, path)
     |> Console.log;
-    Printf.sprintf("set %s %s;", Config.FNM_DIR.name, fnmDir) |> Console.log;
+    Printf.sprintf("set -gx %s %s;", Config.FNM_DIR.name, fnmDir) |> Console.log;
     Printf.sprintf(
-      "set %s %s",
+      "set -gx %s %s",
       Config.FNM_NODE_DIST_MIRROR.name,
       nodeDistMirror,
     )