cirrus-ci: upgrade stack

Author: elopez

Diff for: .cirrus.yml

@@ -16,6 +16,7 @@ task:
   install_dependencies_script: brew install automake libtool haskell-stack gmp
   install_secp256k1_script: .github/scripts/install-libsecp256k1.sh
   install_libff_script: .github/scripts/install-libff.sh
+  install_stack_script: stack upgrade
   install_ghc_script: stack --resolver=lts-20.05 setup
 
   build_dependencies_script: >