Add nightly GitHub Release for `tip` builds

[perbellinioa]

Goal

Publish nightly Ghostty tip .deb packages directly as a GitHub prerelease, making it easy for users to install the latest nightly build without relying on the Launchpad nightly PPA (ppa:mkasberg/ghostty-ubuntu-nightly).

Current Behavior

• install.sh installs from the latest stable GitHub Release in mkasberg/ghostty-ubuntu.
• build-binary/build-ghostty.sh can already build Ghostty from upstream tip:

  build-binary/build-ghostty.sh tip

• .github/workflows/binary-build.yml already supports:

  version_type: tip

• .github/workflows/nightly.yml already builds tip, but only publishes to the nightly PPA:

  ppa:mkasberg/ghostty-ubuntu-nightly
  

There is currently no GitHub Release for nightly builds. Users who want a nightly .deb must either use the PPA or manually download artifacts from GitHub Actions runs.

Proposal

Add a dedicated GitHub Actions workflow .github/workflows/nightly-github-release.yml that:

1. Runs on a nightly schedule (and/or manual workflow_dispatch).
2. Calls .github/workflows/binary-build.yml with version_type: tip.
3. Downloads the generated .deb artifacts.
4. Publishes them to a GitHub Release tagged nightly.

Important caveats

• The nightly release must be a prerelease and must not be marked as latest, so that install.sh and other tooling that fetches the latest release continue to point to stable versions.
• The PPA publishing step is not required unless we also want to keep publishing to Launchpad in parallel.

Optional Enhancement

Add an install-nightly.sh script (similar to install.sh) that installs the latest nightly prerelease via curl, e.g.:

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/mkasberg/ghostty-ubuntu/HEAD/install-nightly.sh)"

[mkasberg]

making it easy for users to install the latest nightly build without relying on the Launchpad nightly PPA

Why would this be a common desire or need?

[mkarklins]

This is more from personal experience, but for me I've had two instances where a bug that I'm encountering is fixed in the "tip" but i have to wait a month or two before the next release lands, so I can get the fixes. I did do a manual build to solve it, but having nightlies available would be "nice to have".

[perbellinioa]

@mkasberg
Sorry for the late reply - for whatever reason I did not get any notification of your comments and I was not regularly checking the issue (as you can see :) ).

I was looking for a feature that was recently released and add to do a manual build. So I thought that having a nightly build that point at tip would have been nice and would have saved me some times. That's basically the reason for the Issue and the PR associated.