Revert ".github: temporarily disable Infix Classic builds"

This reverts commit 2bcb02d3ef and thus
reeneables classic builds again.  Since we build on /mnt on the runner
there should be enough room also for them.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

.github/workflows/build.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         platform: [aarch64, x86_64]
-        variant: [netconf]
+        variant: [netconf, classic]
       fail-fast: false
     steps:
       - name: Maintenance