Attempt to fix coverage builds by using checkout@v2 instead of v1 which might fix the detacthed HEAD issue.
On the off chance it doesn't, add extra logging around git specifically.
We keep getting compat reports for warnings in various compiler
versions. While we can keep merging PRs to resolve these warnings, it
would be nice if the users of other compilers or compiler versions weren't
blocked on us fixing this.
As such, this change disables Werror by default and only enables it when
requested, which happens in CI in test builds.
