[macOS] Fix COPY step on macOS 15 Sequoia (#884)

macOS build fails on Sequoia beta 2 and later (verified in public beta 3
as well) hosts during COPY step when copying files with the `restricted`
extended attribute set.

The problem appears to be due to the `-a` (archive) option, which is an
alias for `-RpP`. Of these, the `-p` option appears to be the source of
the problem. This flag attempts to preserve file metadata including
Access Control Lists and Extended Attributes. However, the `restricted`
extended attribute is not settable when System Integrity Protection
(SIP) is enabled, which it is by default.

This appears to be a change in the implementation of `cp` in macOS 15
Sequoia beta 2 and later. Previous versions would fail to copy the
attribute, but continue without failing. The current version appears to
fail if it cannot set any extended attribute.

Two alternatives to the previous `cp -af` step:
* Use `cp -Rf`. This succeeds but file metadata such as
  creation time, modification time, etc. are not preserved.
* Use `rsync -a`. rsync's -a flag also copies metadata but on a
  best-efforts basis and doesn't error out when it fails to set the
  restricted attribute.

Of the two, the latter more closely approximates the previous behaviour.

The `rm -rf` step can be replaced with `rsync`'s `--delete` option; with
the default `--delete-before` behaviour, this replicates the function of
the `rm -rf`.

Fixes: flutter/flutter#152978

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build/toolchain/mac/BUILD.gn

@@ -222,7 +222,7 @@ template("mac_toolchain") {
     }
 
     tool("copy") {
-      command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
+      command = "ln -f {{source}} {{output}} 2>/dev/null || (rsync -a --delete {{source}} {{output}})"
       description = "COPY {{source}} {{output}}"
     }