ExternalProject: omit --no-checkout from git clone when using git 2.20.x.

The optimization from commit 627fc5b44f (ExternalProject: Avoid
unnecessary checkout on clone, 2019-07-29, v3.16.0-rc1~325^2) triggers a
bug in the Git 2.20.x series that is not in older or newer versions.
Drop the optimization for that specific range of Git versions.

Fixes: #21009
1 file changed