muchsync: set XAPIAN_CONFIG to fix cross compilation

When cross compiling, muchsync needs to be given an explicit
location for Xapian; this commit does that.  Without it, cross
compilation fails with:

```
> checking for xapian-config... no
> configure: error: Cannot find xapian-config
For full logs, run 'nix log /nix/store/gajxjvvi8mizb1zgahrnlcwpw2lapky7-muchsync-aarch64-unknown-linux-gnu-6.drv'.
```

pkgs/applications/networking/mailreaders/notmuch/muchsync.nix

@@ -13,6 +13,7 @@ stdenv.mkDerivation rec {
   };
   nativeBuildInputs = [ pkg-config ];
   buildInputs = [ notmuch openssl sqlite xapian zlib ];
+  XAPIAN_CONFIG = "${xapian}/bin/xapian-config";
   meta = {
     description = "Synchronize maildirs and notmuch databases";
     homepage = "http://www.muchsync.org/";