ci: replicate BPF CI changes for clang installation

Add ability to install specified version of Clang. This replicates what
was done in https://github.com/libbpf/ci/pull/86.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

.github/actions/build-selftests/build_selftests.sh

@@ -8,9 +8,26 @@ source ${THISDIR}/helpers.sh
 
 foldable start prepare_selftests "Building selftests"
 
-LLVM_VER=17
 LIBBPF_PATH="${REPO_ROOT}"
 
+llvm_default_version() {
+	echo "16"
+}
+
+llvm_latest_version() {
+	echo "17"
+}
+
+LLVM_VERSION=$(llvm_default_version)
+if [[ "${LLVM_VERSION}" == $(llvm_latest_version) ]]; then
+	REPO_DISTRO_SUFFIX=""
+else
+	REPO_DISTRO_SUFFIX="-${LLVM_VERSION}"
+fi
+
+echo "deb https://apt.llvm.org/focal/ llvm-toolchain-focal${REPO_DISTRO_SUFFIX} main" \
+	| sudo tee /etc/apt/sources.list.d/llvm.list
+
 PREPARE_SELFTESTS_SCRIPT=${THISDIR}/prepare_selftests-${KERNEL}.sh
 if [ -f "${PREPARE_SELFTESTS_SCRIPT}" ]; then
 	(cd "${REPO_ROOT}/${REPO_PATH}/tools/testing/selftests/bpf" && ${PREPARE_SELFTESTS_SCRIPT})
@@ -24,9 +41,9 @@ fi
 
 cd ${REPO_ROOT}/${REPO_PATH}
 make \
-	CLANG=clang-${LLVM_VER} \
-	LLC=llc-${LLVM_VER} \
-	LLVM_STRIP=llvm-strip-${LLVM_VER} \
+	CLANG=clang-${LLVM_VERSION} \
+	LLC=llc-${LLVM_VERSION} \
+	LLVM_STRIP=llvm-strip-${LLVM_VERSION} \
 	VMLINUX_BTF="${VMLINUX_BTF}" \
 	VMLINUX_H=${VMLINUX_H} \
 	-C "${REPO_ROOT}/${REPO_PATH}/tools/testing/selftests/bpf" \