Feat: setup.sh: Allow overriding which Python binary to use for venv

Debian systems do not have Python3 as `python`. With this change developers on
that platform can use:

```console
$ PYTHON=python3 ./scripts/setup.sh
```

scripts/setup.sh

@@ -13,7 +13,7 @@ if [ -d "$env_name" ]; then
   echo "Virtual environment '$env_name' already exists."
 else
   echo "Creating Virtual environment..."
-  python -m venv .venv
+  ${PYTHON:-python} -m venv .venv
 fi
 echo "Activating virtual environment..."
 source .venv/bin/activate