switch to checkout@master

[jepler]

To see whether we will be compatible with actions/checkout#176

[jepler]

we don't want to take this in the current form. Rather, we should give feedback to actions/checkout based on the outcome. Later, they intend to add this functionality in checkout@v2, and that is the change we should take.

[jepler]

Thie CI shows failures, but actually it's a success: The one sub-task I changed to checkout@master succeeded, and then succeeded again on "re-run all tasks". Another sub-task using checkout@v1 encountered the failure to checkout on re-run that has been vexing us for some time.

Once checkout@v2 is tagged to include the current checkout@master, we should update all our checkout@v1 to checkout@v2. I'll leave this PR open and rebase it later.

[dhalbert]

The last time I looked 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, we need to add:

with:
  fetch-depth: 0

See #2683 (comment) for some more discussion.

[jepler]

@dhalbert thanks, I'll include that when I update this PR. Any idea whether we can turn the missing information (tags?) into an overt error at build time?

[jepler]

The related bug is marked closed:

Thanks all for the feedback! I updated V2 to include the recent submodule/SSH changes from master.
I will update this PR soon to use checkout@v2 with submodules:true and fetch-depth:0 for all checkouts.