oh-my-fish: fix omf-install script

Passing placeholder to substituteAll or replaceVars doesn't work as
expected, it will use the substituteAll's out path instead of the
"caller".

pkgs/by-name/oh/oh-my-fish/package.nix

@@ -4,7 +4,7 @@
   fetchFromGitHub,
   fish,
   runtimeShell,
-  substituteAll,
+  replaceVars,
 }:
 
 stdenv.mkDerivation (finalAttrs: {
@@ -38,14 +38,16 @@ stdenv.mkDerivation (finalAttrs: {
     cp -vr * $out/share/oh-my-fish
 
     cp -v ${
-      substituteAll {
-        name = "omf-install";
-        src = ./omf-install;
-        omf = placeholder "out";
+      replaceVars ./omf-install {
         inherit fish runtimeShell;
+        # replaced below
+        omf = null;
       }
     } $out/bin/omf-install
 
+    substituteInPlace $out/bin/omf-install \
+      --replace-fail '@omf@' "$out"
+
     chmod +x $out/bin/omf-install
     cat $out/bin/omf-install