devops: speedup initial browser checkout (#4352)
devops: speedup initial browser checkout (#4352)

Instead of checking out the whole repository, we now do a shallow
clone.

We then gradually "unshallow" the clone, looking for the `BASE_REVISION`.

This should fix experimental mac-11 builder.
microsoft/playwright,microsoft/playwright,microsoft/playwright,microsoft/playwright,microsoft/playwright,microsoft/playwright,microsoft/playwright,microsoft/playwright,microsoft/playwright