fix source root locations.

These have changed following commit
c3255fe8ec.

See also:

- https://github.com/NixOS/nixpkgs/pull/31004
- https://github.com/NixOS/nixpkgs/pull/31106

pkgs/tools/text/zimreader/default.nix

@@ -23,7 +23,9 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
   buildInputs = [ automake autoconf libtool zimlib cxxtools tntnet ];
-  setSourceRoot = "cd openzim-*/zimreader; export sourceRoot=`pwd`";
+  setSourceRoot = ''
+    sourceRoot=$(echo */zimreader)
+  '';
   preConfigure = "./autogen.sh";
 
   meta = {

pkgs/tools/text/zimwriterfs/default.nix

@@ -28,7 +28,9 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ automake autoconf libtool pkgconfig ];
   buildInputs = [ file icu libgumbo lzma zimlib zlib xapian ];
-  setSourceRoot = "cd openzim-*/zimwriterfs; export sourceRoot=`pwd`";
+  setSourceRoot = ''
+    sourceRoot=$(echo */zimwriterfs)
+  '';
   preConfigure = "./autogen.sh";
 
   meta = {