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