Handle Access Denied error when cleaning up build directory on Windows

[jonsimantov]

Description

Provide details of the change, and generalize the change in the PR title above.

Some of the build artifacts on Windows end up as read-only files, which shutil.rmtree doesn't handle by default. Pass in an onerror handler to make the files writable and try again.

This causes the disk space usage of build_testapps on Windows desktop to drop from 11GB+ to 2.5GB.

---

Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Tested in #959 and in this PR's integration tests.

---

Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

---

Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

Integration test with FLAKINESS (succeeded after retry)

Requested by @jonsimantov on commit 0fa2eba
Last updated: Mon May 16 20:21 PDT 2022
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   FirestoreIntegrationTest.AuthWorks

Add flaky tests to go/fpl-cpp-flake-tracker

[sunmou99]

This is an interesting solution. I saw similar problems before, and I just added os.chmod before the code that is causing the problem.