cmd/exec: convert slashes of explicit bundles (#5219)

The incoming relative path is now converted as follows:

1. filepath.Abs:     foo/bar          -> c:\a\b\c\foo\bar
2. filepath.ToShasl: c:\a\b\c\foo\bar -> c:/a/b/c/foo/bar
3. prepend `file://`

That's something the runtime can make sense of, and the smoke test passes.

Fixes #5134.

Includes:
* ci: add smoke test with a bundle

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>