ci(nss): fix typo USE_SYSTEM USE_SYSTEM_NSS #1194

Job	Run time
Build Firefox (ubuntu-latest, debug)	1h 37m 11s
Build Firefox (ubuntu-latest, release)	1h 35m 41s
Build Firefox (macos-latest, debug)	1h 7m 25s
Build Firefox (macos-latest, release)	1h 26m 40s
Build Firefox (windows-latest, debug)	2h 21m 29s
Build Firefox (windows-latest, release)	2h 10m 23s
Comment on PR	12s
	10h 19m 1s

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