Add nvm installation to install step. #148

Job	Run time
CI test (make validate)	1m 2s
Code coverage (codecov)	0s
Build moodle-plugin-ci.phar	29s
Integration tests (8.3, main, true)	7m 16s
Integration tests (8.3, MOODLE_404_STABLE, true)	6m 55s
Integration tests (8.2, MOODLE_403_STABLE, true)	6m 50s
Integration tests (8.2, MOODLE_402_STABLE, true)	7m 17s
Integration tests (8.1, MOODLE_401_STABLE, true)	7m 2s
Integration tests (8.0, MOODLE_400_STABLE)	5m 58s
Integration tests (8.0, MOODLE_311_STABLE)	5m 33s
Integration tests (7.4, MOODLE_39_STABLE)	5m 4s
SelfUpdate tests (PHAR) (8.3)	44s
SelfUpdate tests (PHAR) (8.2)	50s
Integration tests (PHAR) (8.3, main, true)	6m 45s
SelfUpdate tests (PHAR) (8.1)	43s
Integration tests (PHAR) (8.3, MOODLE_404_STABLE, true)	6m 32s
SelfUpdate tests (PHAR) (8.0)	32s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE, true)	6m 34s
SelfUpdate tests (PHAR) (7.4)	14s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE, true)	5m 33s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE, true)	7m 7s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	5m 25s
	1h 34m 25s

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