fira: Fix permissions of installed files

No need for the executable permissions on fonts.

pkgs/data/fonts/fira/default.nix

···
   installPhase = ''
     runHook preInstall
 
-    mkdir -p $out/share/fonts/opentype
-    cp otf/*.otf $out/share/fonts/opentype
+    install --mode=-x -Dt $out/share/fonts/opentype otf/*.otf
 
     runHook postInstall
   '';