Various cleanups to CI

[gwynne]

• Don't need the BUILDING_DOCC hack (and thus the extra exports check CI job) anymore.
• MASSIVELY simplify the projectboard workflow.
• Reenable CI coverage for the main nightly snapshot since the bug that was crashing the compiler's been fixed.

[codecov-commenter]

Codecov Report

Merging #359 (5a9e152) into main (dd1b740) will not change coverage. The diff coverage is n/a.

:exclamation: Current head 5a9e152 differs from pull request most recent head 1f6ea95. Consider uploading reports for the commit 1f6ea95 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #359 +/- ## ======================================= Coverage 42.56% 42.56% ======================================= Files 117 117 Lines 9695 9695 ======================================= Hits 4127 4127 Misses 5568 5568 ```