Don't run commands relative to Rake task location

What does this MR do and why?

While cutting %15.4, @kpaizee got an error about accessing non-existent files in /lib/tasks/.

When the Rake task moved in !2975 (merged), the working directory moved so we don't want to set it anymore or we want to set it back a couple of levels. I've gone with the former option here.

How to set up and validate locally

If you check out this branch and run ./bin/rake "release:single[15.4]" without a connection to to the internet (so the push can't happen), you should get the same result as: 4b277abd.

Or you could pretend to cut a future version and let the push happen, and delete the new branch when you're finished.

CC @axil

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