Use `kernel.build_dir` by default for proxy dir and metadata cache #1779

Job	Run time
PHPUnit (7.4, highest, stable, false)	30s
PHPUnit (8.0, highest, stable, false)	23s
PHPUnit (8.1, highest, stable, false)	30s
PHPUnit (8.2, highest, stable, false)	25s
PHPUnit (lowest, stable, 7.4)	31s
PHPUnit (5.4.*, highest, 8.0)	28s
PHPUnit (6.*, highest, 8.3)	25s
PHPUnit (8.3, highest, stable, true)	22s
PHPUnit (8.3, highest, dev, true)	21s
PHPUnit (8.3, highest, dev)	24s
Upload coverage to Codecov	0s
	4m 19s

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