ci/container: define LLVM_VERSION in the alpine container job

Instead of allowing defining it in the job, but then not doing that.

The alternative being to delete only the dead `${LLVM_VERSION:=` and `}`
parts, but this way allows for the next commit.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30657>

.gitlab-ci/container/gitlab-ci.yml

@@ -346,6 +346,7 @@ alpine/x86_64_build:
     - .alpine/x86_64_build-base
   variables:
     MESA_IMAGE_TAG: &alpine-x86_64_build ${ALPINE_X86_64_BUILD_TAG}
+    LLVM_VERSION: 18
   rules:
     # Note: the next three lines must remain in that order, so that the rules
     # in `linkcheck-docs` catch nightly pipelines before the rules in `pages`