fix assets release

[deitch]

The assets release workflow was breaking, see https://github.com/lf-edge/eve/actions/runs/4840159952

The cause was that the checkout action was happening after some of the assets were created. checkout can wipe out the contents of the current directory.

This does several things:

1. Moves checkout to the first step
2. Removes the rm -rf assets && mkdir -p assets to its own step, so it is very clear when and where it is happening
3. Adds assets/ to .gitignore, so we do not have any issues with an apparently dirty git repo after checkout and then creating things in assets.
4. Uses the existing checkout in the step Pull the EVE release from DockerHUB or build it rather than trying to check out again
5. Ensures that the one step that did a cd assets (Pull the EVE release from DockerHUB or build it) works out of root and explicitly paths things to assets/

[eriknordmark]

LGTM

Is there a way to test the assets and publish workflow, which only run on master and release branches, short of merging the changes?

[deitch]

Nope. I've wanted that for a long time. I've even asked contacts at GitHub over the years. No such luck.