rav1e: fix `strip` option mangling on x86_64-darwin

`rav1e-0.7.1` slightly changes `strip` option construction in
https://github.com/xiph/rav1e/commit/43fb165e5e07ac047dd59d9d2278bed5da95deb4

Update mangling part to restore stripping.

pkgs/tools/video/rav1e/default.nix

···
   # Darwin uses `llvm-strip`, which results in link errors when using `-x` to strip the asm library
   # and linking it with cctools ld64.
   postPatch = lib.optionalString (stdenv.isDarwin && stdenv.isx86_64) ''
-    substituteInPlace build.rs --replace 'cmd.arg("-x")' 'cmd.arg("-S")'
+    substituteInPlace build.rs --replace-fail '.arg("-x")' '.arg("-S")'
   '';
 
   checkType = "debug";