firebase-ios-sdk

replace set-env with github env file workflow (#6945)

Suggested workaround for security vulnerability listed at,
https://github.com/actions/toolkit/security/advisories/GHSA-mfwh-5m23-j46w

This error can be spotted in the logs currently,
The `set-env` command is deprecated and will be disabled on November 16th. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/