Changes

testenv: fix TESTENV_INSTALL_DIR for --binary-repo

When --binary-repo is set, "/" must be used as install directory,
without any suffix for the distribution.

Fix for:
  realpath: /-debian-trixie/usr/lib/osmo-s1gw: No such file or directory

Fixes: 2152d512 ("testenv: use separate cache dirs per distro")
Change-Id: I1e1ca65fd6a7d77c751dd7138015ff474f8ae576

testenv: fix missing hasattr check

Fix that commands without a --distro argument ("./testenv.py init osmo-dev"
and "./testenv.py clean") fail with:

  AttributeError: 'Namespace' object has no attribute 'distro'

Fixes: 8d21add6 ("testenv: check for --distro without --podman")
Change-Id: I138fda5f5807588428bba4eb1593b887fe7357c9