notmuch-bower: Add runtime dependencies

Bower shells out to several programs, but only two that I could find are
not runtime configurable; "file" for determining mime types and base64
for base64 encoding.

This replaces the shell invocations with the Nix versions of these
programs respectively

pkgs/applications/networking/mailreaders/notmuch-bower/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, mercury, pandoc, ncurses, gpgme }:
+{ lib, stdenv, fetchFromGitHub, mercury, pandoc, ncurses, gpgme, coreutils, file }:
 
 stdenv.mkDerivation rec {
   pname = "notmuch-bower";
@@ -12,6 +12,10 @@ stdenv.mkDerivation rec {
   };
 
   nativeBuildInputs = [ mercury pandoc ];
+  postPatch = ''
+    substituteInPlace src/compose.m --replace 'shell_quoted("base64' 'shell_quoted("${coreutils}/bin/base64'
+    substituteInPlace src/detect_mime_type.m --replace 'shell_quoted("file' 'shell_quoted("${file}/bin/file'
+  '';
 
   buildInputs = [ ncurses gpgme ];