Declare and assign variable separately in setup script

Coverage of `readonly` was added to ShellCheck rule SC2155:

https://github.com/koalaman/shellcheck/wiki/SC2155#problematic-code-in-the-case-of-readonly

> ### Problematic code in the case of `readonly`:
>
> ```sh
> readonly foo="$(mycmd)"
> ```
>
> #### Correct code:
>
> ```sh
> foo="$(mycmd)"
> readonly foo
> ```

There was this sort of "problematic code" in the action setup script, which caused the "Lint shell scripts" CI workflow
to start failing after the 0.7.2 release of ShellCheck.

The script's usage of `readonly` is now adjusted according to ShellCheck's recommendation.

action-setup.sh

@@ -5,14 +5,15 @@
 readonly PYTHON_PACKAGE_VERSION='3.8'
 
 # https://stackoverflow.com/a/29835459
-readonly SCRIPT_PATH="$(
+SCRIPT_PATH="$(
   CDPATH='' \
   cd -- "$(
     dirname -- "$0"
   )" && (
     pwd -P
   )
 )"
+readonly SCRIPT_PATH
 
 readonly PYTHON_COMMAND="python${PYTHON_PACKAGE_VERSION}"
 readonly PYTHON_VENV_PATH="${SCRIPT_PATH}/compilesketches/.venv"