Fix matrix_key ref (#20)

The json was using `"matrix_key"` as a string literal instead of referencing the `matrix_key` variable. This PR fixes the issue.

Author: ahatzz11

index.js

@@ -62,7 +62,7 @@ ${error}`;
     core.setOutput('result', JSON.stringify(outputs_struct))
 
     if (!isEmptyInput(outputs)) {
-        const artifact_content = isEmptyInput(matrix_key) ? outputs_struct : { matrix_key: outputs_struct }
+        const artifact_content = isEmptyInput(matrix_key) ? outputs_struct : { [matrix_key]: outputs_struct }
 
         fs.writeFileSync("./" + step_name, JSON.stringify(artifact_content));
         const fileBuffer = fs.readFileSync("./" + step_name);
@@ -86,4 +86,4 @@ ${error}`;
     }
 } catch (error) {
     core.setFailed(error.message);
-}
+}