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.
```
1 file changed
tree: d62e3690b9a98a7520be171736f08978db5c1782
  1. .github/
  2. code-of-conduct.md
  3. CONTRIBUTING.md
  4. DCO
  5. LICENSE
  6. README.md
  7. rhcos-toolbox
README.md

toolbox - bring your tools with you

rhcos-toolbox is a small script, designed for RHEL CoreOS, that launches a podman container to let you bring in your favorite debugging or admin tools.

This project is deprecated and in maintenance mode.

We recommend that you use https://github.com/containers/toolbox instead.