transaction: Release bundle GPG data as GBytes

BundleData retains explicit GPG key data with g_bytes_ref(), but its
cleanup treats the data as a GObject. This crashes transaction cleanup
when a bundle is installed with explicit key bytes.

Use g_clear_pointer() with g_bytes_unref() to release the retained data.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f