Fixup code coverage config in CI

[gwynne]

• swift-codecov-action now works around the Swift 5.8 problem
• We don't want the unittests flag or need verbose output
• We shouldn't fail CI if coverage uploads fail (it happens often)

[codecov-commenter]

Codecov Report

Merging #362 (0ed1d77) into main (e4cc928) will decrease coverage by 0.52%. The diff coverage is n/a.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #362 +/- ## ========================================== - Coverage 42.51% 41.99% -0.52% ========================================== Files 117 117 Lines 9694 8091 -1603 ========================================== - Hits 4121 3398 -723 + Misses 5573 4693 -880 ``` [see 117 files with indirect coverage changes](https://app.codecov.io/gh/vapor/postgres-nio/pull/362/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vapor)