xrdp: fix clipboard for non-ASCII characters

Without this line, attempting to copy and paste non-ASCII characters
will result in error messages like the following (and pasting from the
server to the client will not work):

```
CLIPBOARD  clipboard_send_data_response_for_text: 823 : ERROR: clipboard_send_data_response_for_text: bad string
```

nixos/modules/services/networking/xrdp.nix

@@ -26,6 +26,12 @@ let
     substituteInPlace $out/sesman.ini \
       --replace LogFile=xrdp-sesman.log LogFile=/dev/null \
       --replace EnableSyslog=1 EnableSyslog=0
+
+    # Ensure that clipboard works for non-ASCII characters
+    sed -i -e '/.*SessionVariables.*/ a\
+    LANG=${config.i18n.defaultLocale}\
+    LOCALE_ARCHIVE=${config.i18n.glibcLocales}/lib/locale/locale-archive
+    ' $out/sesman.ini
   '';
 in
 {