Allow skipping Xcode locator runs

[keith]

For builds that require Xcode, running xcode_locator during toolchain setup is potentially duplicative. The only things it's used for is to check that they exist if it's expected, and to add to the allowed include directories. The include directories already includes /Applications which is where most people likely have Xcode installed. This saves a few seconds when the toolchain is reconfigured.

https://bazelbuild.slack.com/archives/CD3QY5C2X/p1652819625121889