Update disabled setenv command

[hsubox76]

Was causing error in CI:

Run echo "::set-env name=FIREBASE_CI_TEST_START_TIME::$(date +%s)"
Error: Unable to process command '::set-env name=FIREBASE_CI_TEST_START_TIME::1607107014' successfully.
Error: The `set-env` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

See https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

[changeset-bot]

⚠️ No Changeset found

Latest commit: 2e30c1c

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[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (1b54073) and head commit (d1596de).

Test Logs

• Base (1b540737): https://github.com/firebase/firebase-js-sdk/actions/runs/401345734
• Head (d1596dec): https://github.com/firebase/firebase-js-sdk/actions/runs/401374355

[hiranya911]

LGTM