firefox: fix flaky dylib check due to SIGPIPE race

The `otool ... | grep -q ...` pipeline sometimes fails with
PIPESTATUS=141 because of SIGPIPE race.

It happens because `grep -q` doesn't drain the pipe but exits on first
match. If it exists before `otool` writes its last line to stdout,
`otool` fails on write. It makes the whole dylib check fail. This
results in a symlink left pointing to a dylib file instead of copying
it, which may break the app.

The solution is to drain the pipeline with `grep -F`.

pkgs/applications/networking/browsers/firefox/wrapper.nix

@@ -434,7 +434,7 @@ let
                 ;;
               *)
                 # Copy if the symlink resolves to a Mach-O dylib
-                otool -l "$file" 2>/dev/null | grep -q 'LC_ID_DYLIB' || continue
+                otool -l "$file" 2>/dev/null | grep -F 'LC_ID_DYLIB' >/dev/null || continue
                 ;;
             esac