github actions: consider using checkout@v2

[dhalbert]

I am running into issues with PR #2666, which is repeatedly failing job re-runs due to "reference is not a tree" errors, even after merging from upstream. This is discussed in actions/checkout#23, which seems to indicate that we should upgrade to checkout@v2. I will try that in #2666.

[dhalbert]

Oh blah, submodules: true is not supported in checkout@v2 yet. It's just being tested now: actions/checkout#176. Reverting try in #2666 (which maybe just might fix my own build problems).

We should try this again after actions/checkout#176 succeeds.

[dhalbert]

Note also that checkout@v2 by default does a --depth 1 clone, which doesn't work for us, because we use the nearest tag for version information.

To do a full clone, you do:

with:
  fetch-depth: 0

Alternatively, to make sure all the tags are fetched, see https://github.com/actions/checkout/blob/master/README.md#fetch-all-tags. I think that might work and still avoid fetching some of the history.

I spent some time researching whether one could do a shallow clone and then fetch only the tag information, but that's not really how git works.

Another possibility might be to have an initial job before all the board builds that does a complete clone and figures out the version info, and then passes that to the board builds, I guess in an environemtn variable. If they can't find a tag, they would use the info in the env var. That would save some clone time in the board jobs. But I don't think the clone time is all that significant compared to the actual build, so it's not clear it's worth the effort.

[dhalbert]

Fixed by #2765.