emacs: patch mailcap.el to find pkgs.mailcap (#387185)

pkgs/applications/editors/emacs/make-emacs.nix

@@ -46,6 +46,7 @@
   libxml2,
   llvmPackages_14,
   m17n_lib,
+  mailcap,
   mailutils,
   makeWrapper,
   motif,
@@ -244,6 +245,14 @@ mkDerivation (finalAttrs: {
       done
     ''
 
+    ''
+      substituteInPlace lisp/net/mailcap.el \
+        --replace-fail '"/etc/mime.types"' \
+                       '"/etc/mime.types" "${mailcap}/etc/mime.types"' \
+        --replace-fail '("/etc/mailcap" system)' \
+                       '("/etc/mailcap" system) ("${mailcap}/etc/mailcap" system)'
+    ''
+
     ""
   ];