deps: update npm to 6.12.0

Update npm to 6.12.0. Release notes:

6.12.0 (2019-10-08):

Now npm ci runs prepare scripts for git dependencies, and respects the --no-optional argument. Warnings for engine mismatches are printed again. Various other fixes and cleanups.

BUG FIXES

• 890b245dc #252 ci: add dirPacker to options (@claudiahdz)
• f3299acd0 #257 npm.community#4792 warn message on engine mismatch (@ruyadorno)
• bbc92fb8f #259 npm.community#10288 Fix figgyPudding error in npm token (@benblank)
• 70f54dcb5 #241 doctor: Make OK more consistent (@gemal)

FEATURES

• ed993a29c #249 Add CI environment variables to user-agent (@isaacs)
• f6b0459a4 #248 Add option to save package-lock without formatting Adds a new config --format-package-lock, which defaults to true. (@bl00mber)

DEPENDENCIES

• 0ca063c5d npm-lifecycle@3.1.4:
  • fix: filter functions and undefined out of makeEnv (@isaacs)
• 5df6b0ea2 libcipm@4.0.4:
  • fix: pack git directories properly (@claudiahdz)
  • respect no-optional argument (@cruzdanilo)
• 7e04f728c tar@4.4.12
• 5c380e5a3 stringify-package@1.0.1 (@isaacs)
• 62f2ca692 node-gyp@5.0.5 (@isaacs)
• 0ff0ea47a npm-install-checks@3.0.2 (@isaacs)
• f46edae94 hosted-git-info@2.8.5 (@isaacs)

TESTING

• 44a2b036b #262 fix root-ownership race conditions in meta-test (@isaacs)