[conan/ci] Enforce Xcode 11.7 selection
[conan/ci] Enforce Xcode 11.7 selection

For now, binary packages are not available in Conan center for Xcode 12.0

jrl-umi3218/RBDyn,jrl-umi3218/RBDyn,jrl-umi3218/RBDyn,jrl-umi3218/RBDyn