Add nvm installation to install step. #690

Job	Run time
CI test (make validate)	1m 49s
Code coverage (codecov)	1m 3s
Build moodle-plugin-ci.phar	52s
Integration tests (8.3, main, true)	7m 20s
Integration tests (8.3, MOODLE_404_STABLE, true)	6m 52s
Integration tests (8.2, MOODLE_403_STABLE, true)	6m 20s
Integration tests (8.2, MOODLE_402_STABLE, true)	6m 5s
Integration tests (8.1, MOODLE_401_STABLE, true)	8m 1s
Integration tests (8.0, MOODLE_400_STABLE)	5m 50s
Integration tests (8.0, MOODLE_311_STABLE)	5m 31s
Integration tests (7.4, MOODLE_39_STABLE)	5m 2s
SelfUpdate tests (PHAR) (8.3)	41s
Integration tests (PHAR) (8.3, main, true)	6m 8s
SelfUpdate tests (PHAR) (8.2)	34s
Integration tests (PHAR) (8.3, MOODLE_404_STABLE, true)	6m 4s
SelfUpdate tests (PHAR) (8.1)	23s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE, true)	6m 10s
SelfUpdate tests (PHAR) (8.0)	29s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE, true)	5m 41s
SelfUpdate tests (PHAR) (7.4)	50s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE, true)	6m 28s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 26s
	1h 33m 39s

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