pants-plugins/uses_services: respect `ST2_DATABASE__*` vars #5546

Job	Run time
Skip Duplicate Jobs Pre Job	4s
Unit Tests (chunk 1) - Python 3.8	6m 58s
Lint Checks (black, flake8, etc.) - Python 3.8	2m 53s
Unit Tests (chunk 2) - Python 3.8	5m 41s
Compile (pip deps, pylint, etc.) - Python 3.8	3m 59s
Pack Tests - Python 3.8	2m 48s
Unit Tests (chunk 1) - Python 3.9	7m 14s
Self-check on Python 3.8 - Python 3.8	8m 27s
Integration Tests (chunk 1) - Python 3.8	5m 10s
Lint Checks (black, flake8, etc.) - Python 3.9	3m 1s
Unit Tests (chunk 2) - Python 3.9	5m 37s
Integration Tests (chunk 2) - Python 3.8	6m 11s
Compile (pip deps, pylint, etc.) - Python 3.9	3m 53s
Pack Tests - Python 3.9	2m 47s
Integration Tests (chunk 1) - Python 3.9	5m 17s
Integration Tests (chunk 2) - Python 3.9	6m 22s
Slack notification for failed master builds	3s
	1h 16m 25s

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