GitHub Action: fix Windows build
GitHub Action: fix Windows build

Summary:
Watchman's GitHub Action has been failing because of the use of `set-env` is deprecated due to security issues. See https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/ for details.

See https://github.com/facebook/watchman/runs/1520165260?check_suite_focus=true for example of failures.

To fix it we just need to write to the special environment file.

Reviewed By: chadaustin

Differential Revision: D25412442

fbshipit-source-id: d31ef025189c8aebfa08d434d9373f5da7d4690e

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro