This changes npm.cmd and the npm script when running in Cygwin to look
for an npm installation in prefix, running it if found. The default npm
is used to get the prefix. This implements the changes described in
https://github.com/joyent/node/issues/8528 .
Fixes joyent/node#8528. Reference: joyent/node#8554.
Fixes: #6412
PR-URL: https://github.com/npm/npm/pull/9089