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

[jailuthra]

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:

go-livepeer/cmd/livepeer_bench/livepeer_bench.go

Line 106 in 3908252

u := path.Join(dir, v.URI)