ci: Require LLVM_VERSION to be set explicitly

Stop falling back to 15 in random places; make sure it's set clearly
every time we want to use it.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/33137>

.gitlab-ci/container/build-skqp.sh

@@ -68,8 +68,8 @@ cat "${SKQP_PATCH_DIR}"/build-skqp_*.patch |
 
 # hack for skqp see the clang
 pushd /usr/bin/
-ln -s "../lib/llvm-${LLVM_VERSION:-15}/bin/clang" clang
-ln -s "../lib/llvm-${LLVM_VERSION:-15}/bin/clang++" clang++
+ln -s "../lib/llvm-${LLVM_VERSION}/bin/clang" clang
+ln -s "../lib/llvm-${LLVM_VERSION}/bin/clang++" clang++
 popd
 
 # Fetch some needed build tools needed to build skia/skqp.