How to install latest node-canvas from custom mirror

I'm working on a project that uses `node-canvas` at work, but our build system is closed off from the open internet, so we're required to mirror binaries like `node-canvas` internally. Lots of other packages support providing an environment variable to point to an alternative mirror host or path and I'm hoping that `node-canvas` does too, or can.

I found [another issue](https://github.com/Automattic/node-canvas/issues/1346#issuecomment-479689564) from a few years ago, that seems mostly targeted to the now deprecated `node-canvas-prebuilt` version. The tldr; being that an env variable named `canvas_binary_host_mirror` should have the mirror path in it and allow downloading from there, but from my testing that isn't recognized at all.

None of the installation instructions I can find shed any light on whether this is still supported in some form, so I was hoping it is and is just not documented yet. Or there's a process that can get support for this added.

## Your Environment
* Version of node-canvas (output of `npm list canvas` or `yarn list canvas`): 2.7.0
* Environment (e.g. node 4.2.0 on Mac OS X 10.8): Node 14.16.0 on Ubuntu