fix(build): read tip version from tarball VERSION file

[psadi]

Problem

Nightly (tip) builds started failing:

+ cd ghostty-1.3.2-main+5b435d5-build.zig.zon.txt
./bin/build-ghostty.sh: line 72: cd: ghostty-1.3.2-main+5b435d5-build.zig.zon.txt: No such file or directory

Failing run: https://github.com/pkgforge-dev/ghostty-appimage/actions/runs/36660278577

Root cause

The tip version was parsed by splitting the tarball's top-level directory on hyphens and taking fields 2–4:

ghostty-1.3.2-main+5b435d5/build.zig.zon.txt
  -> 1.3.2-main+5b435d5-build.zig.zon.txt

Upstream changed the tip version scheme from main-<hash> to semver build metadata main+<hash> (Config.zig: .pre = branch, .build = short_hash). The hash merged into the branch field, so field 4 became the file name and produced an invalid version.

This cascaded into cat ghostty-<bogus>/build.zig.zon failing (leaving ZIG_VERSION empty and printing the misleading Zig is already installed), then cd ghostty-<bogus> failing under set -e.

Fix

Ghostty embeds a canonical VERSION file at <tarball-root>/VERSION, so read the version directly from it instead of parsing the directory name. Also fail fast on empty detection, and use --no-wildcards-match-slash so only the top-level VERSION is matched (nested files can never leak in).

Verification

• Tested against the live tip tarball: resolves 1.3.2-main+26e64df, matching the extracted directory.
• Full tip-branch flow simulated: tarball rename → VERSION write → extract → cd target exists → ZIG_VERSION=0.16.0.
• sh -n and pre-commit (shellcheck, shfmt) pass.