google-fonts: Turn into a fixed-output derivation

Also remove the executable bit from some fonts, and don't build on
Hydra because this package is huge.

pkgs/data/fonts/google-fonts/default.nix

···
   phases = [ "unpackPhase" "installPhase" ];
 
   installPhase = ''
-    mkdir -p $out/share/fonts/truetype
-    find . -name "*.ttf" -exec cp -v {} $out/share/fonts/truetype \;
+    dest=$out/share/fonts/truetype
+    mkdir -p $dest
+    find . -name "*.ttf" -exec cp -v {} $dest \;
+    chmod -x $dest/*.ttf
   '';
 
+  outputHashAlgo = "sha256";
+  outputHashMode = "recursive";
+  outputHash = "0q03gg0sh2mljlbmhamnxz28d13znh9dzca84p554s7pwg6z4wca";
+
   meta = with stdenv.lib; {
     homepage = https://www.google.com/fontsl;
     description = "Font files available from Google Font";
     license = with licenses; [ asl20 ofl ufl ];
     platforms = platforms.all;
+    hydraPlatforms = [];
     maintainers = with maintainers; [ manveru ];
   };
 }