CI: Avoid unneeded autopkgtest --apt-pocket parameter

This is to mitigate error such as this:
```
Running setup script /usr/share/autopkgtest/setup-commands/setup-testbed...
awk: fatal: cannot open file `/etc/apt/sources.list' for reading: No such file or directory
sh: Attempting to set up Debian/Ubuntu apt sources automatically
```

We're building the network-manager package anyways, so don't need the
--apt-pocket=proposed=src:network-manager parameter.

.github/workflows/network-manager.yml

@@ -70,5 +70,4 @@ jobs:
           sudo autopkgtest -U \
             --copy=debian/artifacts:/root/ --setup-commands='dpkg -i /root/*.deb' \
             --env=DEB_BUILD_OPTIONS=nocheck \
-            --apt-pocket=proposed=src:network-manager \
             network-manager_*.dsc -- lxd autopkgtest/ubuntu/noble/amd64 || test $? -eq 2  # allow for skipped tests (exit code = 2)