ci: fix LLVM 20 build

Seems like LLVM 20 had a release.

Signed-off-by: Andrii Nakryiko <[email protected]>

Author: anakryiko

.github/docker/Dockerfile.ubuntu

@@ -8,7 +8,7 @@ ARG SHORTNAME="jammy"
 ENV SHORTNAME=$SHORTNAME
 
 RUN apt-get update && apt-get install -y curl gnupg
-RUN if [ "${LLVM_VERSION}" = "20" ]; \
+RUN if [ "${LLVM_VERSION}" = "21" ]; \
     then \
         echo "\n\
 deb http://apt.llvm.org/${SHORTNAME}/ llvm-toolchain-${SHORTNAME} main\n\