🏗 Reduce coverage threshold for codecov check

[rsimha]

Background:

• There appears to be a problem with our codecov check, where it reports incorrect coverage levels for PRs from old branches: Codecov reporting total-repo coverage #28212 (comment)
• In addition, there doesn't seem to be a straightforward way to override the baseline for comparisons: Codecov reporting total-repo coverage #28212 (comment) and Patch Status should compare to merge-base instead of HEAD codecov/codecov-bash#83 (comment)
• Meanwhile, it appears that the total project-level coverage was reduced from 80.5% to ~76-77% by f26a825. (It's currently unknown why this was so.)

PR highlights:

This PR makes a few changes to temporarily work around the issues described above and to enable better debugging:

• Reduce project-level threshold from 80% to 75%
• Apply coverage status immediately after the coverage report is uploaded (don't wait for entire Travis build to complete)
• Don't require entire CI build to pass

Future work:

Continue debugging and figure out if there's a workaround for codecov issue codecov/codecov-bash#83 (comment), and consider using a different service if this continues to be a dealbreaker.

Related to #28212

[amp-owners-bot]

Hey @estherkim! These files were changed:

build-system/pr-check/local-tests.js

[rcebulko]

LGTM; hopefully we can figure out diff coverage soon, that would make a big difference (no pun intended)