chore: Bump Node version to 12.18.4

Doing this as suggested by @scinos to make sure all the changes in this
branch build under the this version, which is what master has been using
since eea8ef6 (#45721).

This is an extra caution step, but a useful one nonetheless.

Given the merge strategy for PRs in Calypso at the time of writing is
"Squash and merge" this change won't even make it into `master`, as it's
already there.

Normally, I would rebase the branch on top of the base branch to work
around this kind of issues, but given this is a release branch it would
not be appropriate.

.nvmrc

@@ -1 +1 @@
-12.18.0
+12.18.4