windows ci: Use Actions checkout

[nerdvegas]

Currently, the Windows docker image entrypoint performs a rez repo checkout in order to run the tests, even though this has already been done by the 'Checkout' task in the GH workflow.

Sometimes an error occurs related to this (I think this may be happening on forked PRs, since they will result in a reference to a commit that doesn't exist on the nerdvegas repo yet):

Cloning into 'rez'...
14
fatal: reference is not a tree: 6b63c81dbb54a78b49b7bafb19e02b50e185def8
15
##[error]Process completed with exit code 1.

Instead of checking out the rez repo, the docker image should instead bind mount the already checked out clone.

[nerdvegas]

Closed by https://github.com/nerdvegas/rez/releases/tag/2.47.14