Fix handling of existing imports in containerd

[elezar]

This change fixes the handling of existing imports in containerd config files. Without this fix the existing imports were dropped and replaced with the import directive for the generated drop-in file.

Before this change:

$ cat test-config.toml
imports = ["/foo/bar/*.toml"]
version = 2

Run:

nvidia-ctk runtime configure --runtime=containerd --config=test-config.toml --drop-in-config=$(pwd)/99-nvidia.toml

$  cat test-config.toml
imports = ["/Users/elezar/dev/container-toolkit/*.toml"]
version = 2

With this change:

$ cat test-config.toml
imports = ["/foo/bar/*.toml", "/Users/elezar/dev/container-toolkit/*.toml"]
version = 2

[elezar]

/cherry-pick release-1.18

[tariq1890]

LGTM. Thanks @elezar !

[github-actions]

🤖 Backport PR created for release-1.18: #1384 ✅