Yarn fails to install dependencies

This is bug 1733718. It should be fixed in snapcraft >= 2.36, but xenial-updates only has 2.35.

We've discussed switching Launchpad over to use the snap, and Sergio said that their team was going to attempt that (I gave guidance), but I haven't heard anything since.

Sergio, in the meantime, could we please have an updated SRU for xenial?

Really busy at the moment with an unrelated new release of the CLI, but I had a few issues I wanted to let you guys know about.

* It seems the snap has stopped even attempting to be built on builds.snapcraft.io for some reason. Last build was a week ago but we've published commits to master since then

* The yarn node plugin adds dirty changes to the git repo, I think in the yarn.lock. You can see this even in the builds.snapcraft.io builds

* I would like to be able to pass extra args to yarn, like --network-concurrency 1 to test for issues.

* I was able to build the snap just fine locally, but because it said the git version was dirty I didn't want to so I ended up writing a nil plugin: https://github.com/heroku/cli/blob/master/snap/snapcraft.yaml

* The nil plugin I wrote quickly as possible, but it might be a good starting point for a conventional way to build plugins. Note how I fetch the needed node binary from the SNAPCRAFT_IMAGE_INFO architecture env var.

* Because I'm looking at the SNAPCRAFT_IMAGE_INFO env var this could be easily done cross-compiled. I don't need to run on 32-bit or ARM—that env var just needs to be set.

* Right now I was only able to get the 64-bit version of the snap out. We have lots of users on 32-bit and ARM so I know it's only a matter of time until we get users asking for that. It's definitely more my problem than your guys, if I had to I can get some vms setup to build the snaps but would much prefer to use builds.snapcraft.io for all of this!

Thanks for the hard work! Despite these issues I really love working on the platform and the tools you guys are putting together are fantastic!

> It seems the snap has stopped even attempting to be built on builds.snapcraft.io for some reason. Last build was a week ago but we've published commits to master since then

Note that the build farm was disabled for a while due to Meltdown/Spectre. It's starting to come back now, but in pieces, and it has a large backlog now. More information here:

    https://forum.snapcraft.io/t/note-build-snapcraft-io-partially-restored-see-below-for-details/3480