build: create a symlink to src/buildtools during sync

[dsanders11]

Description of Change

We use CHROMIUM_BUILDTOOLS_PATH pretty aggressively, but recent upstream changes have made it clear that they consider it "highly unsupported" so we need to switch to a better method.

@codebytere put up electron/build-tools#855, and I agree with the general approach that symlinking it is the cleanest solution we have to work with depot_tools/gclient_paths.py. However, that PR would only work for folks using build-tools, and we also need to support folks who are building without build-tools and bring their own depot_tools.

This PR puts the symlink creation in as part of the sync process, so it will always exist whether you're building with build-tools or not. I'll follow up with a PR for build-tools to detect if the symlink exists and matches CHROMIUM_BUILDTOOLS_PATH, and if so not set the environment variable, otherwise continue to set it for backwards compatibility. After enough time has passed that we're confident all supported builds are using the changes in this PR, we can rip out CHROMIUM_BUILDTOOLS_PATH from build-tools as well.

Checklist

• I have built and tested this change
• I have filled out the PR description
• I have reviewed and verified the changes

Release Notes

Notes: none