automation: remove allow_failure from Alpine Linux jobs

Now that the Alpine Linux build jobs complete successfully on staging we
can remove the "allow_failure: true" tag.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Wei Liu <wl@xen.org>

automation/gitlab-ci/build.yaml

@@ -434,25 +434,21 @@ alpine-3.12-gcc:
   extends: .gcc-x86-64-build
   variables:
     CONTAINER: alpine:3.12
-  allow_failure: true
 
 alpine-3.12-gcc-debug:
   extends: .gcc-x86-64-build-debug
   variables:
     CONTAINER: alpine:3.12
-  allow_failure: true
 
 alpine-3.12-clang:
   extends: .clang-x86-64-build
   variables:
     CONTAINER: alpine:3.12
-  allow_failure: true
 
 alpine-3.12-clang-debug:
   extends: .clang-x86-64-build-debug
   variables:
     CONTAINER: alpine:3.12
-  allow_failure: true
 
 
 # Arm builds