www/node12: Update 12.17.0 -> 12.18.0

https://nodejs.org/en/blog/release/v12.18.0/

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

MFH: 2020Q2
Security: 11fcfa8f-ac64-11ea-9dab-000d3ab229d6
Sponsored by: Miles AS