[Chapel Merge] Rename "Full" checks to "Nightly", and stop running per-PR

Branch: refs/heads/main
Revision: 82d6f9e5b1e49592b1498be51b32b55e430cc155
Author: arifthpe
Link: Rename "Full" checks to "Nightly", and stop running per-PR by arifthpe · Pull Request #29332 · chapel-lang/chapel · GitHub
Log Message:
Rename "Full" checks to "Nightly", and stop running per-PR (#29332)

Change the Full checks tier to only run nightly and on demand, removing
the per-push to main trigger, and accordingly change its name to
Nightly.

This is because we're hitting concurrent runner limits in GA pretty
hard, and would rather have those resources available for robust
pre-merge checks long term.

[reviewed by @jabraham17 , thanks!]

Compare: Comparing 56bcfa5187835717e559e652cda75ce51342789c...52a4c2ee8eeb9f3fbd8a4e68ec336a0ec18d25b5 · chapel-lang/chapel · GitHub

Diff:
R091 .github/workflows/trigger-full-checks.yml .github/workflows/trigger-nightly-checks.yml
https://github.com/chapel-lang/chapel/pull/29332.diff