Use `kernel.build_dir` by default for proxy dir and metadata cache #1780

Job	Run time
PHPUnit (7.4, highest, stable, false)	29s
PHPUnit (8.0, highest, stable, false)	41s
PHPUnit (8.1, highest, stable, false)	25s
PHPUnit (8.2, highest, stable, false)	29s
PHPUnit (lowest, stable, 7.4)	21s
PHPUnit (5.4.*, highest, 8.0)	28s
PHPUnit (6.*, highest, 8.3)	23s
PHPUnit (8.3, highest, stable, true)	29s
PHPUnit (8.3, highest, dev, true)	26s
PHPUnit (8.3, highest, dev)	32s
Upload coverage to Codecov	0s
	4m 43s

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