poppler_min: pin to poppler_0_61

poppler_min is only used by texlive.bin, which doesn't build
with poppler 0.64 and blocked the update poppler 0.63->0.64.
Pin to poppler_0_61 (which we currently need for libreoffice)
so we don't have to keep 0_63 as a third version.
See discussion in #40327 and #40180

pkgs/top-level/all-packages.nix

@@ -11011,7 +11011,10 @@ with pkgs;
     introspectionSupport = true;
   });
 
-  poppler_min = poppler.override { # TODO: maybe reduce even more
+  poppler_min = poppler_0_61.override { # TODO: maybe reduce even more
+    # this is currently only used by texlive.bin.
+    # pinned to 0.61 because texlive 2017 doesn't build w/poppler 0.64
+    # TODO: review poppler version for texlive 2018
     minimal = true;
     suffix = "min";
   };