fix: support cleaning root output directory in DownloadActionOutputs (#667)

When an action specifies that the entire working directory is an output
directory (dir.Path is "" or "."), relPath evaluates to ".".
Attempting to call root.RemoveAll(".") fails with "RemoveAll .: invalid argument"
(syscall.EINVAL) because os.Root cannot delete its own root directory.

Add a removeAllUnder helper function that normalizes relPath with filepath.Clean,
and if relPath resolves to ".", removes all entries inside root instead.

Extend the existing TestDownloadActionOutputs to include an output directory
with Path "" to prevent regressions.

Signed-off-by: Yaowen Mei <ywmei@google.com>