home-environment: generate checked activation script

By using `writeShellScript` when writing the activation script we
automatically get the right shebang and also get syntax validation.

Issue #2015

modules/home-environment.nix

@@ -541,9 +541,7 @@ in
         ]
         + optionalString (!cfg.emptyActivationPath) "\${PATH:+:}$PATH";
 
-        activationScript = pkgs.writeScript "activation-script" ''
-          #!${pkgs.runtimeShell}
-
+        activationScript = pkgs.writeShellScript "activation-script" ''
           set -eu
           set -o pipefail