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

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

···
   libxml2,
   llvmPackages_14,
   m17n_lib,
+  mailcap,
   mailutils,
   makeWrapper,
   motif,
···
       for makefile_in in $(find . -name Makefile.in -print); do
         substituteInPlace $makefile_in --replace-warn /bin/pwd pwd
       done
+    ''
+
+    ''
+      substituteInPlace lisp/net/mailcap.el \
+        --replace-fail '"/etc/mime.types"' \
+                       '"/etc/mime.types" "${mailcap}/etc/mime.types"' \
+        --replace-fail '("/etc/mailcap" system)' \
+                       '("/etc/mailcap" system) ("${mailcap}/etc/mailcap" system)'
     ''
 
     ""