Use `kernel.build_dir` by default for proxy dir and metadata cache (#… #1783

Job	Run time
PHPUnit (7.4, highest, stable, false)	29s
PHPUnit (8.0, highest, stable, false)	24s
PHPUnit (8.1, highest, stable, false)	22s
PHPUnit (8.2, highest, stable, false)	33s
PHPUnit (lowest, stable, 7.4)	30s
PHPUnit (5.4.*, highest, 8.0)	34s
PHPUnit (6.*, highest, 8.3)	34s
PHPUnit (8.3, highest, stable, true)	24s
PHPUnit (8.3, highest, dev, true)	25s
PHPUnit (8.3, highest, dev)	27s
Upload coverage to Codecov	13s
	4m 55s

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