r_session is leaking file descriptors

[gaborcsardi]

At least on Unix, possibly on Windows as well:

❯ rs <- callr::r_session$new()

❯ rs$run(function() 1+1)
[1] 2

❯ rs$run(function() ps::ps_num_fds(ps::ps_handle()))
[1] 8

❯ rs$run(function() 1+1)
[1] 2

❯ rs$run(function() ps::ps_num_fds(ps::ps_handle()))
[1] 12

They are probably simply not closed in the post-hook.