Log dist-url.
Log dist-url.

The default download URL can be overridden by `--dist-url` or one of the
`*_MIRROR` environment variables.  Log the URL to ease troubleshooting.

Fixes: https://github.com/nodejs/node-gyp/issues/1169
PR-URL: https://github.com/nodejs/node-gyp/pull/1170
Reviewed-By: Refael Ackermann <0604a90ccdde7a0adfa2ff3857349859a7209740@gmail.com>

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