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/alpine/x86_64_build.sh

@@ -8,8 +8,6 @@
 set -e
 set -o xtrace
 
-export LLVM_VERSION="${LLVM_VERSION:=18}"
-
 EPHEMERAL=(
 )
 

.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`

.gitlab-ci/image-tags.yml

@@ -32,7 +32,7 @@ variables:
    DEBIAN_TEST_VK_TAG: "20240813-piglit-1d"
    KERNEL_ROOTFS_TAG: "20240813-piglit-1d"
 
-   ALPINE_X86_64_BUILD_TAG: "20240808-llvm"
+   ALPINE_X86_64_BUILD_TAG: "20240814-var"
    ALPINE_X86_64_LAVA_SSH_TAG: "20240401-wlproto"
    FEDORA_X86_64_BUILD_TAG: "20240717-pyyaml"
    KERNEL_TAG: "v6.6.21-mesa-f8ea"