lib/shell: add appendToVar function

modules/home-environment.nix

@@ -579,8 +579,9 @@ in
         ${config.lib.shell.exportAll cfg.sessionVariables}
       '' + concatStringsSep "\n"
         (mapAttrsToList
-          (env: values: ''
-            export ${env}="''$${env}''${${env}:+:}${concatStringsSep ":" values}"'')
+          (env: values: config.lib.shell.export
+            env
+            (config.lib.shell.appendToVar ":" env values))
           cfg.sessionSearchVariables)
         + cfg.sessionVariablesExtra;
     };

modules/lib/shell.nix

@@ -1,6 +1,14 @@
 { lib }:
 
 rec {
+  # Produces a Bourne shell like statement that appends new values to
+  # an possibly existing variable, using sep(ator).
+  # Example:
+  #   appendToVar ":" "PATH" [ "$HOME/bin" "$HOME/.local/bin" ]
+  #   => "$PATH\${PATH:+:}$HOME/bin:$HOME/.local/bin"
+  appendToVar = sep: n: v:
+    "\$${n}\${${n}:+${sep}}${lib.concatStringsSep sep v}";
+
   # Produces a Bourne shell like variable export statement.
   export = n: v: ''export ${n}="${toString v}"'';