sage: refactor passthru on sage-env

`writeTextFile` supports `passthru` today, so let's use it.

The previous solution could cause eval errors in some obscure override
scenarios - so obscure, that I can't provide a simple reproducer. But, I
did hit it during some experimentation.

pkgs/by-name/sa/sage/sage-env.nix

@@ -102,6 +102,12 @@ in
 writeTextFile rec {
   name = "sage-env";
   destination = "/${name}";
+
+  passthru = {
+    lib = sagelib;
+    docbuild = sage-docbuild;
+  };
+
   text =
     ''
       export PKG_CONFIG_PATH='${
@@ -205,8 +211,3 @@ writeTextFile rec {
           }''${DYLD_LIBRARY_PATH:+:}$DYLD_LIBRARY_PATH"
       '';
 }
-// {
-  # equivalent of `passthru`, which `writeTextFile` doesn't support
-  lib = sagelib;
-  docbuild = sage-docbuild;
-}