fix(tool): make copied frameworks writable to fix macOS 15.4+ openrsync regression (#189658)

### Description

This PR fixes a regression of #147142 where read-only Flutter SDK builds
fail on macOS 15.4+.

### Root Cause Analysis

Starting in macOS 15.4+, Apple transitioned from GNU `rsync` to
BSD-licensed `openrsync`. A major behavioral difference is that
**`openrsync` silently ignores the `--chmod` flag** during local
directory synchronization.

As a result, the `--chmod` parameter passed to `rsync` during framework
copy operations is ignored, and framework binaries inherit the read-only
permissions from the read-only SDK cache (e.g. Nix store or system-wide
read-only path). Subsequent `lipo` and codesigning operations fail with
a `Permission denied` error.

### Solution

This PR adds an explicit `chmod -R u+w` execution on the copied
framework/xcframework target path immediately after `rsync` copy
completes to ensure writability, completely decoupling file writability
from `rsync`'s specific parameters.

Fixes #189645