Inject unresolved symlinks with BwoB

[fmeum]

Creating unresolved symlinks locally when building with --remote_download_outputs=minimal results in unnecessary I/O and may not even be supported by the host (e.g. on Windows). Instead, only create it in the remote action file system.

[tjgq]

Thanks for looking into this. I'd prefer if we simply changed RAFS#createSymbolicLink to only create the symlink in the in-memory component (instead of adding a separate injection API). I'm curious to know if you tried that approach but ran into trouble elsewhere? (One issue is that we can't use RAFS in SymlinkTreeHelper since they would never be materialized on disk, but that already needs to change in light of #24867.)

[fmeum]

I would prefer that approach, in particular because it would also resolve (or at least help with) #21747. Should I give that a try in this PR? I remember that we weren't certain whether that would be safe.

[tjgq]

I would prefer that approach, in particular because it would also resolve (or at least help with) #21747. Should I give that a try in this PR? I remember that we weren't certain whether that would be safe.

Let's give it a try in a separate PR and see what breaks? We can then reevaluate whether to attempt to fix whatever does, or come back to this one.

[fmeum]

I sent #24897 and will set the current PR to Draft.