Remove remaining `allowSubstitutes = false`

See, e.g., https://github.com/NixOS/nix/issues/4442 for wider
discussions.

home-manager/default.nix

@@ -16,7 +16,6 @@ let
 
 in runCommand "home-manager" {
   preferLocalBuild = true;
-  allowSubstitutes = false;
   meta = with lib; {
     description = "A user environment configurator";
     maintainers = [ maintainers.rycee ];

home-manager/install.nix

@@ -3,7 +3,6 @@
 runCommand "home-manager-install" {
   propagatedBuildInputs = [ home-manager ];
   preferLocalBuild = true;
-  allowSubstitutes = false;
   shellHookOnly = true;
   shellHook = ''
     confFile="''${XDG_CONFIG_HOME:-$HOME/.config}/nixpkgs/home.nix"

modules/home-environment.nix

@@ -652,7 +652,6 @@ in
           "home-manager-generation"
           {
             preferLocalBuild = true;
-            allowSubstitutes = false;
           }
           ''
             mkdir -p $out

modules/programs/fish.nix

@@ -310,7 +310,6 @@ in {
             nativeBuildInputs = [ pkgs.python2 ];
             buildInputs = [ cfg.package ];
             preferLocalBuild = true;
-            allowSubstitutes = false;
           } ''
             mkdir -p $out
             if [ -d $src/share/man ]; then

tests/modules/programs/firefox/profile-settings.nix

@@ -32,7 +32,6 @@ with lib;
         firefox-unwrapped = pkgs.runCommand "firefox-0" {
           meta.description = "I pretend to be Firefox";
           preferLocalBuild = true;
-          allowSubstitutes = false;
           passthru.gtk3 = null;
         } ''
           mkdir -p "$out/bin"

tests/modules/programs/firefox/state-version-19_09.nix

@@ -13,7 +13,6 @@ with lib;
         firefox-unwrapped = pkgs.runCommand "firefox-0" {
           meta.description = "I pretend to be Firefox";
           preferLocalBuild = true;
-          allowSubstitutes = false;
           passthru.gtk3 = null;
         } ''
           mkdir -p "$out/bin"