Fix fortanix build

Author: Mark-Simulacrum

src/ci/docker/host-x86_64/dist-various-2/build-x86_64-fortanix-unknown-sgx-toolchain.sh

@@ -7,7 +7,7 @@ target="x86_64-fortanix-unknown-sgx"
 
 install_prereq() {
     curl https://apt.llvm.org/llvm-snapshot.gpg.key|apt-key add -
-    add-apt-repository -y 'deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic main'
+    add-apt-repository -y 'deb http://apt.llvm.org/bionic/ llvm-toolchain-bionic-11 main'
     apt-get update
     apt-get install -y --no-install-recommends \
             build-essential \