nixos/printing: fix CUPS SetEnv directive placement

With CUPS v2.3b5, the configuration directive `SetEnv`
moved from `cupsd.conf` to `cups-files.conf`.  See also
https://github.com/apple/cups/commit/d47f6aec436e0e9df6554436e391471097686ecc .

We have to follow up as `SetEnv` is now ignored in `cupsd.conf`.
Without this, executables called by cups
can't find other executables they depend on,
like `gs` or `perl`.

nixos/modules/services/printing/cupsd.nix

@@ -60,6 +60,8 @@ let
 
     TempDir ${cfg.tempDir}
 
+    SetEnv PATH /var/lib/cups/path/lib/cups/filter:/var/lib/cups/path/bin
+
     # User and group used to run external programs, including
     # those that actually send the job to the printer.  Note that
     # Udev sets the group of printer devices to `lp', so we want
@@ -76,8 +78,6 @@ let
     '') cfg.listenAddresses}
     Listen /var/run/cups/cups.sock
 
-    SetEnv PATH /var/lib/cups/path/lib/cups/filter:/var/lib/cups/path/bin
-
     DefaultShared ${if cfg.defaultShared then "Yes" else "No"}
 
     Browsing ${if cfg.browsing then "Yes" else "No"}