Fix call from gdb.debug to qemu.ld_prefix

[f0rki]

gdb.debug calls qemu.ld_prefix with env as first parameter. The signature of ld_trace is

def ld_prefix(path=None, env=None):

So env is passed as path. This results in path being an enironment dictionary, which is breaking launching qemu --help to get the LD_PREFIX in this line.

[f0rki]

Is the CI generally broken? From the error message it seems to be some other problem?