Use nightlies on new Fortress branches

[chapulina]

Part of gazebo-tooling/release-tools#428.

Some of these are not strictly necessary yet, some of them may never be necessary. Nevertheless, I find it easier to have a rule that we apply to all main branches rather than having to analyze case by case. It shouldn't hurt that ign-cmake3 loads nightlies even though it doesn't use (for now) anything from there.

I propose the following workflow:

1. Beginning of development cycle: all main branches use nightlies
2. Feature freeze: as pre-releases start to be made, swap nightly for prerelease
3. Code freeze: after stable releases are made, bump all main branches to the next versions and go back to 1. ☝️

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Update

Reverted the idea above for now, so this PR has only the branches that are known to go into Fortress. I think that having a more standard approach across the main branches will save us time, but I understand that this may cause confusion and not everyone is onboard, so I'm shelving this idea for now.