sandbox: Open host file for read-only access if needed

sandbox: Open host file for read-only access if needed

Some files cannot be written but read-only access is still useful for
tests. Add a fallback to read-only access when needed.

This is useful in CI when opening a large data file provided by docker,
where read/write access would result in copying the file, thus needing
a lot of extra disk space.

Signed-off-by: Simon Glass <sjg@chromium.org>