ci: single build-command for ns7zip (Linux has python)

Linux template provides `python`, so the OS-aware split is
unnecessary. Use `python build_plugin.py` for both matrix legs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/build-ns7zip.yml

@@ -31,8 +31,7 @@ jobs:
           # configs by default; --final/--dist-dir do not exist (they were
           # forwarded to the per-version script and rejected). Output DLLs
           # are copied to <repo>/plugins/{x86-ansi,x86-unicode,amd64-unicode}.
-          # Ubuntu 24.04 has no `python`, use `python3` on the Linux leg.
-          build-command:   ${{ matrix.target == 'linux' && 'python3 build_plugin.py' || 'python build_plugin.py' }}
+          build-command:   'python build_plugin.py'
           artifact-source: 'plugins'
           submodules:      'true'
           guest-os:        ${{ matrix.target == 'linux' && 'Linux' || 'Windows' }}