k3s: add bash to runtime deps for tailscale

When setting up k3s to use tailscale as a network backend, the flannel go
library (used by k3s by default) depends on the `sh` binary to execute commands,
in particular to run `PostStartupCommand` and `ShutdownCommand` commands.
Without having bash/sh available, the network setup process will fail with:

```
flannel exited: failed to register flannel network: failed to run command: tailscale set --accept-routes --advertise-routes=$SUBNET Err: exec: \"sh\": executable file not found in $PATH
```

This fixes that. ref #317223