src: fix --icu-data-dir= regression

Commit a8734af4 ("src: make copies of startup environment variables")
from two weeks ago introduced a regression in the capturing of the
`--icu-data-dir=` switch: it captured the string up to the `=` instead
of what comes after it.

PR-URL: https://github.com/nodejs/node/pull/11255


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>