Issue 7076 - Fix relative paths for nodes

[CarycaKatarzyna]

It solves #7076. I decided to change rootdir to invocation dir in _bestrelpathcache, because we want to have path for nodes relative to invocation directory. As I've seen, that function is used only for nodes (_node_location_to_relpath), so this change shouldn't mess other functionalities.

[nicoddemus]

Thanks @CarycaKatarzyna!

because we want to have path for nodes relative to invocation directory

I'm not sure that is correct: the rootdir exists specifically so tests have always reliable nodeids, regardless where pytest was invoked from. The nodeid is after all used by plugins to uniquely identify tests, for example --lf, which persists the node ids to disk; having them change depending on where pytest was invoked from would defeat that.

@RonnyPfannschmidt can you please correct me if I'm wrong?

[RonnyPfannschmidt]

this absolutely has to use rootdir, we cant use invocationdir as things are now

[nicoddemus]

Right thanks.

@CarycaKatarzyna thanks again for the PR, but we will need to decline this as is. Feel free to open another PR if you come up with a different approach! 👍