Use `kernel.build_dir` by default for proxy dir and metadata cache #1777

Job	Run time
PHPUnit (7.4, highest, stable, false)	29s
PHPUnit (8.0, highest, stable, false)	23s
PHPUnit (8.1, highest, stable, false)	1m 0s
PHPUnit (8.2, highest, stable, false)	35s
PHPUnit (lowest, stable, 7.4)	37s
PHPUnit (5.4.*, highest, 8.0)	26s
PHPUnit (6.*, highest, 8.3)	23s
PHPUnit (8.3, highest, stable, true)	23s
PHPUnit (8.3, highest, dev, true)	26s
PHPUnit (8.3, highest, dev)	25s
Upload coverage to Codecov	0s
	5m 7s

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