Only Alpine kernels use the vmlinuz-VARIANT naming scheme, postmarketOS
kernel images are always called vmlinuz.
Since we only use our own kernels now, we can completely get rid of the
variable and always look for vmlinuz.
[ci:skip-vercheck]: the checks can't handle bumping pkgver *and* pkgrel
[ci:skip-build]: takes a long time and already built successfully in CI
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7398>
Aelin
2056701d6c