cabal: cosmetic fix to syntax highlighting in Emacs

pkgs/build-support/cabal/default.nix

···
               fi
 
               ${optionalString (self.enableSharedExecutables && self.isExecutable && self.stdenv.isDarwin) ''
-                for exe in $out/bin/* ; do
+                for exe in "$out/bin/"* ; do
                   install_name_tool -add_rpath \
                     $out/lib/${ghc.ghc.name}/${self.pname}-${self.version} $exe
                 done