fix(ci): run headless input tests without X

[luinbytes]

Why

The v2.0.7 release run stopped before publication because the Linux test process imported pynput without an X display. macOS and Windows completed, but the release job correctly stayed skipped after the Linux failure.

Scope

• Set PYNPUT_BACKEND=dummy only for the release workflow's test step.
• Keep every platform's PyInstaller build on its native input backend.
• Set the package version to 2.0.8 for the next immutable release tag.

The failed v2.0.7 tag remains unchanged and has no GitHub Release.

Blast Radius

The workflow change affects only test discovery in release CI. It does not change packaged input handling or application behavior.

Verification

• The v2.0.7 Linux log failed at pynput with failed to acquire X connection.
• PYNPUT_BACKEND=dummy QT_QPA_PLATFORM=offscreen python -m unittest discover tests -v passed 30 tests with 2 expected platform skips.
• actionlint .github/workflows/release.yml passed.
• YAML read-back confirmed both headless test variables.
• Package-version read-back returned 2.0.8.
• git diff --check passed.

[luinbytes]

PASS+NOTES.

Independent verification covered base 86702817456b3407ee340529910d7c144aef30ab through head 179f5542573df6d9626cc0b637f86ddffdac544a. Stable patch-id is b6a51ff4d4163049a15a0236fe2815e55060d90c.

The two-file patch is limited to the test-step environment and the 2.0.8 package version. PYNPUT_BACKEND=dummy and QT_QPA_PLATFORM=offscreen are scoped to Run tests; the PyInstaller build runs in the following step without those overrides. actionlint passed, and local python -m unittest discover tests -v passed with 30 tests and 2 expected skips.

PR metadata reports MERGEABLE and CLEAN, with no PR checks currently reported. Preflight run 33719877868 is running on the exact head. Ubuntu and macOS jobs are green. The Windows job is still building, so artifact and Windows acceptance remain pending.