Fix ShellCheck errors and warnings

```
In ./rhcos-toolbox line 21:
        source "${TOOLBOXRC}"
               ^------------^ SC1090 (warning): ShellCheck can't follow non-constant source. Use a directive to specify location.
In ./rhcos-toolbox line 48:
    local runlabel=$(image_runlabel)
          ^------^ SC2155 (warning): Declare and assign separately to avoid masking return values.
In ./rhcos-toolbox line 63:
    local state=$(container_state)
          ^---^ SC2155 (warning): Declare and assign separately to avoid masking return values.
In ./rhcos-toolbox line 183:
        echo "${@}; exit" | sudo podman attach "${TOOLBOX_NAME}"
              ^--^ SC2145 (error): Argument mixes string and array. Use * or separate argument.
```