Auto merge of #10931 - ehuss:ignore-hg, r=weihanglo

Always allow hg to be missing on CI.

`hg` isn't installed on rust-lang/rust Docker images, which causes this check to fail.

Rather than trying to carefully enforce the requirements for `hg` being tested, this just ignores the test if it is unavailable on CI. I think this is something that can be revisited if Cargo ever gains more hg support.