The list of disabled repositories was recently converted from a single
file into a directory with separate repository files, so let's adjust
the setup script accordingly.
```
$ ls -lR /etc/apt/sources.list.d/
/etc/apt/sources.list.d/:
total 36
-rw-r--r-- 1 root root 76 Nov 3 10:28 azure-cli.list
-rw-r--r-- 1 root root 72 Nov 3 10:22 bazel.list
drwxr-xr-x 2 root root 4096 Nov 3 10:31 disabled
-rw-r--r-- 1 root root 113 Nov 3 10:13 docker-source.list
-rw-r--r-- 1 root root 367 Nov 3 10:28 github_git-lfs.list
-rw-r--r-- 1 root root 111 Nov 3 10:25 google-chrome-source.list
-rw-r--r-- 1 root root 64 Nov 3 10:14 google-cloud-sdk.list
-rw-r--r-- 1 root root 54 Nov 3 10:23 helm-stable-debian.list
-rw-r--r-- 1 root root 89 Nov 3 10:29 yarn-source.list
/etc/apt/sources.list.d/disabled:
total 20
-rw-r--r-- 1 root root 100 Nov 3 10:23 devel_kubic_libcontainers_stable.list
-rw-r--r-- 1 root root 103 Nov 3 10:27 git.list
-rw-r--r-- 1 root root 105 Nov 3 10:22 gradle.list
-rw-r--r-- 1 root root 118 Nov 3 10:13 pypy.list
-rw-r--r-- 1 root root 104 Nov 3 10:13 python.list
```
case "$phase" in
SETUP)
# remove semaphore repos, some of them don't work and cause error messages
- sudo rm -f /etc/apt/sources.list.d/*
+ sudo rm -rf /etc/apt/sources.list.d/*
# enable backports for latest LXC
echo "deb http://archive.ubuntu.com/ubuntu $UBUNTU_RELEASE-backports main restricted universe multiverse" | sudo tee -a /etc/apt/sources.list.d/backports.list