ipns: fix test by using global random reader

[hacdias]

I think this may fix the test that was intermittently failing on Windows. By using the global crypto/rand.Reader we avoid creating two different random generators seeded with the same time, and therefore likely generating the same key.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.83%. Comparing base (2559ec2) to head (68da08c).

@@            Coverage Diff             @@
##             main     #586      +/-   ##
==========================================
- Coverage   59.94%   59.83%   -0.12%     
==========================================
  Files         232      232              
  Lines       28127    28127              
==========================================
- Hits        16861    16829      -32     
- Misses       9798     9821      +23     
- Partials     1468     1477       +9     

see 8 files with indirect coverage changes

[hacdias]

This is a relatively small change. I will merge in hopes this will indeed fix the issue. We shall see.