automation: Fix the Alpine clang builds to use clang

Looks like a copy&paste error.

Fixes: f6e1d85 ("automation: add alpine linux x86 build jobs")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

automation/gitlab-ci/build.yaml

@@ -443,13 +443,13 @@ alpine-3.12-gcc-debug:
   allow_failure: true
 
 alpine-3.12-clang:
-  extends: .gcc-x86-64-build
+  extends: .clang-x86-64-build
   variables:
     CONTAINER: alpine:3.12
   allow_failure: true
 
 alpine-3.12-clang-debug:
-  extends: .gcc-x86-64-build-debug
+  extends: .clang-x86-64-build-debug
   variables:
     CONTAINER: alpine:3.12
   allow_failure: true