Revert Debian latest version to 5.6.7

The Ubuntu PPA/mainline repository does not yet have v5.6.8, thus builds
targetting it will fail. Revert back to v5.6.7, which is available, and
increment pkgrel.

.github/workflows/debian.yml

@@ -7,8 +7,8 @@ name: Debian
 
 env:
   GPG_KEY_ID: 56C464BAAC421453
-  KERNEL_VERSION: 5.6.8
-  KDEB_PKGVERSION: 5.6.8-1
+  KERNEL_VERSION: 5.6.7
+  KDEB_PKGVERSION: 5.6.7-2
   LOCALVERSION: -surface
 
 jobs: