fix(build): freeze x/exp dependency

[kolesnikovae]

slices.SortFunc from the experimental golang.org/x/exp has changed its signature which breaks in our code, and in dependencies:

golang/exp@302865e

Note that this changes some of the sorting functions to use a
comparison function rather than a less function. We don't promise
backward compatibility in x/exp packages. Being compatible with the
Go 1.21 package seems more useful for people not yet using 1.21,
as it will make the transition to 1.21 easier.

See https://github.com/grafana/pyroscope/actions/runs/6886513285/job/18732588802?pr=2700.

This is a quick fix: I've pinned the latest compatible version of x/exp. We should probably get rid of the dependency and update to Go 1.21 (which has built-in slices package)

Also had to pin x/sys in ebpf submodule due to golang/go#63969

[kolesnikovae]

/cc @korniltsev please take another look: 1e437f0 - turns out x/sys 0.14 is broken as well