Downgrade setup-ocaml to fix Windows CI issue

[cknitt]

setup-ocaml 2.0.11 contains the following change:

Don't install Cygwin's git or mercurial packages (reduces cache by ~90MB).

(see also ocaml/setup-ocaml#648)

In our CI script, we are setting up the git config as follows for Windows:

git config --global core.autocrlf false
git config --global core.eol lf

With the Git Windows client (already installed in the runner image) used instead of Cygwin's git, this causes the following error when running git diff later on in the CI process:

Run git diff --exit-code lib/js lib/es6
fatal: bad boolean config value 'false
' for 'core.autocrlf'

I am therefore pinning the setup-ocaml version to 2.0.10 for now.