Add nvm installation to install step. #691

Job	Run time
Code coverage (codecov)	59s
CI test (make validate)	1m 20s
Build moodle-plugin-ci.phar	1m 34s
Integration tests (8.3, main, true)	7m 0s
Integration tests (8.3, MOODLE_404_STABLE, true)	6m 48s
Integration tests (8.2, MOODLE_403_STABLE, true)	6m 32s
Integration tests (8.2, MOODLE_402_STABLE, true)	6m 21s
Integration tests (8.1, MOODLE_401_STABLE, true)	7m 25s
Integration tests (8.0, MOODLE_400_STABLE)	6m 5s
Integration tests (8.0, MOODLE_311_STABLE)	5m 25s
Integration tests (7.4, MOODLE_39_STABLE)	5m 2s
SelfUpdate tests (PHAR) (8.3)	38s
Integration tests (PHAR) (8.3, main, true)	6m 2s
SelfUpdate tests (PHAR) (8.2)	39s
SelfUpdate tests (PHAR) (8.1)	27s
Integration tests (PHAR) (8.3, MOODLE_404_STABLE, true)	6m 5s
SelfUpdate tests (PHAR) (8.0)	36s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE, true)	5m 59s
SelfUpdate tests (PHAR) (7.4)	13s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE, true)	5m 54s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE, true)	6m 54s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 8s
	1h 33m 6s

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