`continuity.context.Walk` assumes incorrect behavior for `os.Readlink`

[bcmills]

The code added in 8100e75 (CC @darstahl) appears to be assuming a behavior of os.Readlink that it does not actually provide, which will almost certainly cause Walk to fail when the root is a relative symlink or symlink-to-symlink on POSIX platforms.

• (See golang/go#57766.)

A cleaner fix here is to ensure that the root path passed to filepath.Walk and/or filepath.WalkDir ends with a trailing separator, which (on all POSIX-compliant platforms, and Windows as of https://go.dev/cl/463177) will cause Walk to start walking at the target referred to by the root instead of the root symlink itself.

• (See https://go.dev/cl/484475 for an example of such a change.)