opam: fix enableFishIntegration (#3597)

Fish shell doesn't require arguments to `eval` to be double quoted
like in a bash shell. At the moment doing so gives us the following
error:

~/.config/fish/config.fish (line 12): $(...) is not supported. In fish, please use '(/nix/store/8asq…)'.
eval "$(/nix/store/8asqgnhs89wzyjvs8p1n5hvxn7lkn9wa-opam-2.1.3/bin/opam env --shell=fish)"
      ^
from sourcing file ~/.config/fish/config.fish
	called during startup
source: Error while reading file “/home/user/.config/fish/config.fish”

This commit fixes the above error.

modules/programs/opam.nix

@@ -56,7 +56,7 @@ in {
     '';
 
     programs.fish.shellInit = mkIf cfg.enableFishIntegration ''
-      eval "$(${cfg.package}/bin/opam env --shell=fish)"
+      eval (${cfg.package}/bin/opam env --shell=fish)
     '';
   };
 }