Tests: Pass API key to distributed test reporter.

Pass environment variable for test reporter API key to
`docker-compose` so that it can authenticate and submit results
to the hosting distributed tests.

Props SergeyBiryukov, mikeschroder.
Fixes #49846.
Built from https://develop.svn.wordpress.org/trunk@47556


git-svn-id: https://core.svn.wordpress.org/trunk@47331 1a063a9b-81f0-0310-95a4-ce76da25c4cd