Use nightlies for all main branches

[chapulina]

Revisiting an idea which was originally introduced in #31, but eventually reverted.

• Current approach: use nightlies when a main branch is known to go into the upcoming collection
  • Cons:
    • It's one more PR to open as part of a dependency bumping process, which is already complicated as it is
    • We often refrain from adding the nighlies for a main branch while bumping if we know that it won't be immediately necessary. This means we need to track it back when it is necessary, causing more churn. See Use nightlies on new Fortress branches #31
  • Pros:
    • Only use nightlies when strictly necessary, so if we see nightlies enabled, we know they're needed
• Suggestion: use nightlies for all main branches
  • Cons:
    • We lose the nuance of whether nightlies are actually necessary or not
  • Pros:
    • Easier to maintain, we can just do for all branches the moment we fork ign-XN from main, which would be part of the post-release process: https://github.com/ignition-tooling/release-tools/blob/master/.github/ISSUE_TEMPLATE/release_collection.md#post-release