ci: Use -s not --script to run tests

gut's cmdline script accepts a number of command-line arguments. It
appears that the Godot engine removes most of its own options from the
argument list available to user code, but it leaves any `--script FOO`
or `-s FOO` arguments in place.

gut 9.2.1 understands both the `--script` and `-s` forms, but gut 9.3.0
only understands the `-s` form and raises an error if the `--script`
form is used.

bitwes/Gut#667

Author: wjt

.github/workflows/checks.yaml

@@ -42,4 +42,4 @@ jobs:
         godot --path . --headless --import
     - name: Run tests
       run: |
-        godot --path . --headless --script addons/gut/gut_cmdln.gd -gexit
+        godot --path . --headless -s addons/gut/gut_cmdln.gd -gexit

addons/block_code/README.md

@@ -153,15 +153,17 @@ This plugin uses the [Godot Unit Test](https://gut.readthedocs.io/en/latest/) (G
 Tests can also be run from the command line using the GUT command line script:
 
 ```
-godot --path . --headless --script addons/gut/gut_cmdln.gd -gexit
+godot --path . --headless -s addons/gut/gut_cmdln.gd -gexit
 ```
 
 A few options are of note here:
 
 - `--path` instructs Godot to use the project in the current directory.
 - `--headless` instructs Godot to run without a display or sound.
-- `--script` instructs Godot to run the GUT command line script instead of
-  running the main scene.
+- `-s` instructs Godot to run the GUT command line script instead of
+  running the main scene. Due to a [bug in
+  GUT](https://github.com/bitwes/Gut/issues/667), the long form `--script`
+  cannot be used.
 - `-gexit` is an option for the GUT command line script that instructs GUT to
   exit after the tests complete.