www/node12: Update 12.18.3 -> 12.18.4

https://nodejs.org/en/blog/release/v12.18.4/

This is a security release. See
https://nodejs.org/en/blog/vulnerability/september-2020-security-releases/

MFH: 2020Q3
Security: 4ca5894c-f7f1-11ea-8ff8-0022489ad614
Sponsored by: Miles AS