Add nightly cron schedule to E2E workflow

[Swiddis]

Description

Per side-channel discussion, we're looking at re-enabling E2E tests to run nightly (as opposed to on every PR).

Part of improving our testing strategy in response to #1001.

Issues Resolved

N/A

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

Merging #1005 (d32f300) into main (e7bdc67) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             main    #1005    +/-   ##
========================================
  Coverage   43.83%   43.83%            
========================================
  Files         311      311            
  Lines       18676    18676            
  Branches     4555     4565    +10     
========================================
  Hits         8187     8187            
+ Misses      10447     9941   -506     
- Partials       42      548   +506     

Flag	Coverage Δ
dashboards-observability	43.83% <ø> (ø)

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see 79 files with indirect coverage changes