Add maintenance-9.x to nightly build workflow

[sensei-hacker]

Summary

Add maintenance-9.x branch to the nightly build workflow triggers.

This enables automatic pre-release builds when commits are pushed to the maintenance-9.x branch, providing complete firmware artifacts (hex files + SITL binaries) for RC releases.

Problem

Currently, CI runs on PR creation but not on merge. After merging multiple PRs to maintenance-9.x, there's no CI run containing all the merged changes. This makes it impossible to get complete firmware artifacts for RC releases.

Solution

Add maintenance-9.x to the branch list in .github/workflows/nightly-build.yml (alongside master and maintenance-8.x.x).

Changes

• Added maintenance-9.x to the push trigger branches

[github-actions]

Branch Targeting Suggestion

You've targeted the master branch with this PR. Please consider if a version branch might be more appropriate:

• maintenance-9.x - If your change is backward-compatible and won't create compatibility issues between INAV firmware and Configurator 9.x versions. This will allow your PR to be included in the next 9.x release.

• maintenance-10.x - If your change introduces compatibility requirements between firmware and configurator that would break 9.x compatibility. This is for PRs which will be included in INAV 10.x

If master is the correct target for this change, no action is needed.

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This is an automated suggestion to help route contributions to the appropriate branch.