Add relocatable installs to support concurrency-safe cached environments

[charliermarsh]

Summary

The idea here is similar to what we do for wheels: we create the CachedEnvironment in the archive-v0 bucket, then symlink it to its content-addressed location. This ensures that we can always recreate these environments without concern for whether anyone else is accessing them.

Part of the challenge here is that we want the virtual environments to be relocatable, because we're now building them in one location but persisting them in another. This requires that we write relative (rather than absolute) paths to scripts and entrypoints. The main risk with relocatable virtual environments is that the scripts and entrypoints themselves are not relocatable, because they use a relative shebang. But that's fine for cached environments, which are never intended to leave the cache.

Closes #5503.