cgit: use full path of groff in man2html

groff may not be installed system-wide or may be outside cgit's path
(e.g. when running cgit using uwsgi). As a result, the man2html filter
fails. Replace groff by its full path in man2html.

(cherry picked from commit bbcf5eb)

pkgs/applications/version-management/git-and-tools/cgit/default.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchurl, openssl, zlib, asciidoc, libxml2, libxslt
 , docbook_xsl, pkgconfig, luajit
-, gzip, bzip2, xz
+, groff, gzip, bzip2, xz
 , python, wrapPython, pygments, markdown
 }:
 
@@ -32,6 +32,9 @@ stdenv.mkDerivation rec {
         -e 's|"bzip2"|"${bzip2.bin}/bin/bzip2"|' \
         -e 's|"xz"|"${xz.bin}/bin/xz"|' \
         -i ui-snapshot.c
+
+    substituteInPlace filters/html-converters/man2html \
+      --replace 'groff' '${groff}/bin/groff'
   '';
 
   # Give cgit a git source tree and pass configuration parameters (as make