CI: add test result uploads and refactor nightly config

[onetechnical]

Summary

The CI pipeline for PRs and nightly tests were revised. This PR makes the following changes:

• Fix SHORTTEST flag for e2e_subs.
• Re-configure nightlies to better manage concurrency
• Make slack webhook optional
• Add test result uploads
• Limit build uploads to targeted branches
• Add workflow dispatch for nightly jobs

Test Plan

Verified upload of test files for both PRs and nightly runs

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.25%. Comparing base (310a900) to head (ef628b9).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6372      +/-   ##
==========================================
- Coverage   50.60%   50.25%   -0.35%     
==========================================
  Files         653      653              
  Lines      110395   111124     +729     
==========================================
- Hits        55864    55847      -17     
- Misses      51657    52431     +774     
+ Partials     2874     2846      -28     

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[gmalouf]

Added a few questions/comments, but looks reasonable.