Use `kernel.build_dir` by default for proxy dir and metadata cache #1781

Job	Run time
PHPUnit (7.4, highest, stable, false)	36s
PHPUnit (8.0, highest, stable, false)	24s
PHPUnit (8.1, highest, stable, false)	22s
PHPUnit (8.2, highest, stable, false)	24s
PHPUnit (lowest, stable, 7.4)	48s
PHPUnit (5.4.*, highest, 8.0)	25s
PHPUnit (6.*, highest, 8.3)	26s
PHPUnit (8.3, highest, stable, true)	27s
PHPUnit (8.3, highest, dev, true)	35s
PHPUnit (8.3, highest, dev)	24s
Upload coverage to Codecov	12s
	5m 3s

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