deps: upgrade npm to v6.12.1
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
I followed all the steps on https://github.com/nodejs/node/blob/master/doc/guides/maintaining-npm.md#step-5-update-licenses, just like I did in #25234 :-)
Also fixes #30066 (closed), altho it's already closed.