CI: Have nc-autotools use source distribution instead of repository

[DWesl]

Instead of a clone of the repository, have the nc-autotools job work from a source distribution prepared by a previous autotools CI job.

This should catch most of the "files not included in EXTRA_DIST" or similar issues I remember, and probably most of the "netcdf-c does not pass make distcheck" errors.

I'm not sure whether this is in scope for a CI job, but the nc-autotools jobs should be a fairly complete copy of make distcheck now; the only things I can think of not included in that job would be make install and make installcheck. I'm only aware of #2431 raising issues with installed files, so that may be less relevant.

[WardF]

Interesting. There is definitely value here, and no obvious downside other than 18 months down the line "Why are we doing it this way?". It might introduce a little bit of overhead to the already lengthy CI scripts, but as you point out, it would catch extra dist related errors.

I know that the CI triggers can be far more complex than the ones we're using right now; it's entirely possible that we would want this to run as part of the 'one-off' set of tests, and not for every test permutation.

Thanks a lot for this, let me think about how to best integrate it!