Use nvm 0.39.7 #684

Job	Run time
CI test (make validate)	1m 1s
Code coverage (codecov)	1m 2s
Build moodle-plugin-ci.phar	37s
Integration tests (8.3, main, true)	6m 56s
Integration tests (8.3, MOODLE_404_STABLE, true)	6m 23s
Integration tests (8.2, MOODLE_403_STABLE, true)	6m 16s
Integration tests (8.2, MOODLE_402_STABLE, true)	6m 22s
Integration tests (8.1, MOODLE_401_STABLE, true)	6m 58s
Integration tests (8.0, MOODLE_400_STABLE)	5m 53s
Integration tests (8.0, MOODLE_311_STABLE)	5m 4s
Integration tests (7.4, MOODLE_39_STABLE)	5m 10s
Integration tests (PHAR) (8.3, main, true)	5m 48s
SelfUpdate tests (PHAR) (8.3)	32s
Integration tests (PHAR) (8.3, MOODLE_404_STABLE, true)	5m 58s
SelfUpdate tests (PHAR) (8.2)	47s
Integration tests (PHAR) (8.2, MOODLE_403_STABLE, true)	5m 58s
Integration tests (PHAR) (8.1, MOODLE_402_STABLE, true)	5m 50s
SelfUpdate tests (PHAR) (8.1)	26s
Integration tests (PHAR) (8.0, MOODLE_401_STABLE, true)	6m 21s
SelfUpdate tests (PHAR) (8.0)	23s
Integration tests (PHAR) (7.4, MOODLE_400_STABLE)	4m 53s
SelfUpdate tests (PHAR) (7.4)	15s
	1h 28m 53s

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