WARN if local_execution_root_dir is not on the same file system as cache directories

[cburroughs]

Currently if these are on different file systems, everything will appear to work fine on small projects or examples, but:

• Performance will be bad because one can not hardlink (among other possible optimizations) across file systems
• If /tmp is tmpfs (aka just in memory) Pants sandboxes will quickly fill it up.

It is totally reasonable (ex from slack: https://pantsbuild.slack.com/archives/C046T6T9U/p1694679410518159) for people to not be familiar with the details of their CI filesystem layout, so even if this "works" on their workstation, they get bitten with spooky CI problems.

We can steer away from this unlikely-to-be-what-you-want configuration by issuing a WARNing on startup.