Update runfiles preamble to refer to `rules_shell`

[UebelAndre]

Why not update the preamble to refer to rules_shell?
https://github.com/bazelbuild/rules_shell/blob/f28373ff7d291172e293c06b67ab0737da576e4a/shell/runfiles/runfiles.bash#L70-L80

This can cause issues in contexts where runfiles links are not made (e.g. scripts in actions that may use runfiles).

[fmeum]

Could you elaborate on the situations in which you would expect this to break? We keep the file in the same path as before via root_symlinks, simply because we need to maintain backwards compatibility with the old snippet anyway.

[UebelAndre]

This turned out to be somewhat of a misunderstanding where a source file was being consumed and not the executable. But that mix up aside, what would be the damage in having both the symlink and the actual file and updating docs to refer to something like the following?

set -uo pipefail; set +e; f=rules_shell/shell/runfiles/runfiles.bash 

Is it because the name would hard to determine based on the bzlmod generated repository names?

[fmeum]

Yes, that's mostly why. @mering has a PR out to make this change, but it requires a number of cases based on the Bazel version. We could add another root symlink in a fixed path that looks more like rules_shell, but then we would have two runfiles for the same file.

[mering]

See discussion in #21