livepeer_bench: back-slashes in the file path don't work

Describe the bug Back-slashes in the input manifest's path cause the benchmarking to fail on Windows.

To Reproduce Here's the original Discord message from a user reporting this problem.

Expected behavior Back-slashes should work out-of-the-box on Windows, or alternatives (like using forward-slashes) should be better documented.

Screenshots

Desktop (please complete the following information):

• OS: Windows

Additional context Golang's path library does not work well with back-slashes We can try using an alternate library when creating the segment path from the input manifest's directory: https://github.com/livepeer/go-livepeer/blob/3908252d23edd62590c61b02598c5fffc148014b/cmd/livepeer_bench/livepeer_bench.go#L106