public-inbox: add xapian to PATH

Some public-inbox programs run xapian-compact, so we need to make sure
it's in PATH.

nixos/tests/public-inbox.nix

···
     # require to use --all
     machine.succeed("curl -L https://machine.${domain}/inbox/repo1/repo1@root-1/raw | sudo -u public-inbox public-inbox-learn rm --all")
     machine.fail("curl -L https://machine.${domain}/inbox/repo1/repo1@root-1/T/#u | grep 'This is a testing mail.'")
+
+    # Compact the database
+    machine.succeed("sudo -u public-inbox public-inbox-compact --all")
   '';
 })

pkgs/servers/mail/public-inbox/default.nix

···
             --set NIX_CFLAGS_COMPILE_${stdenv.cc.suffixSalt} -I${lib.getDev libxcrypt}/include \
             --prefix PATH : ${lib.makeBinPath [
               git
+              xapian
               /* for InlineC */
               gnumake
               stdenv.cc