libeatmydata: fix launcher script - find shell library properly

The new version of the launcher script in version 105 doesn't have the #8665
bug, but it does try to find the shell library using Debian tools, which
obviously doesn't work on Nix. Removed the now-unneccessary makeWrapper and
patched out the Debian bits.

pkgs/development/libraries/libeatmydata/default.nix

@@ -8,11 +8,10 @@ stdenv.mkDerivation rec {
     sha256 = "1pd8sc73cgc41ldsvq6g8ics1m5k8gdcb91as9yg8z5jnrld1lmx";
   };
 
-  buildInputs = [ makeWrapper ];
-
-  postInstall = ''
-    wrapProgram $out/bin/eatmydata \
-      --prefix PATH : $out/bin
+  patches = [ ./find-shell-lib.patch ];
+  patchFlags = "-p0";
+  postPatch = ''
+    substituteInPlace eatmydata.in --replace NIX_OUT_DIR $out
   '';
 
   meta = {

pkgs/development/libraries/libeatmydata/find-shell-lib.patch

@@ -0,0 +1,20 @@
+--- eatmydata.in	2020-02-01 18:10:59.618679823 -0800
++++ eatmydata.in.new	2020-02-01 18:08:25.092620247 -0800
+@@ -15,15 +15,8 @@
+ # You should have received a copy of the GNU General Public License
+ # along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ 
+-export `dpkg-architecture|grep DEB_BUILD_MULTIARCH`
+-
+-shlib="/usr/lib/$DEB_BUILD_MULTIARCH/eatmydata.sh"
+-if [ -f "$shlib" ]; then
+-    . "$shlib"
+-else
+-    echo "Unable to locate eatmydata shell library, it was not enabled" >&2
+-    exec "$@"
+-fi
++shlib="NIX_OUT_DIR/libexec/eatmydata.sh"
++. "$shlib"
+ 
+ usage()
+ {