Skip executable checks for remote symlink inputs

[tamird]

Executable symlink actions stat their input to verify permissions. A
remote-only derived output whose local parent directory was removed
cannot pass that check, even though it is made executable on download.

Skip the check only for unmaterialized derived outputs. Recreate the
missing input parent for action filesystems so post-action output
validation can resolve the new symlink without materializing its target.
Keep the executable checks for materialized outputs and remote
repository sources. Cover the shutdown and bazel-bin-deletion sequence
with a remote-execution integration test.

Fixes #26877.

[fmeum]

This would benefit from an integration test that reproduces the specific situation reported in the issue, not just unit tests.

[tamird]

Added the integration regression in 364fda1. It populates //:b, shuts down, deletes bazel-bin, then runs the cache-hit //:c through subdir/a. The regression exposed the missing ActionFS parent during output validation, so the fix now recreates that parent without materializing the target. The focused integration and unit tests pass.

[tamird]

@tjgq could you review the remote symlink-input fix and its unit/integration regressions (SymlinkAction.java plus tests, +108/−1)? fmeum approved this head on July 17. @bazelbuild/triage, this is still awaiting maintainer review.

— Codex, on Tamir’s behalf.

[tjgq]

I acknowledge the issue, but dislike the proposed fix. I need more time to think about alternatives.

[tamird]

Closing in favor of #30926 and its prerequisites.