setup-sdl: discriminator option: Add default value

SDL Commits
1 
From a966470e7d9b6a5c93eec3f08d5dac370325c860 Mon Sep 17 00:00:00 2001
From: andy5995 <[EMAIL REDACTED]>
Date: Fri, 15 May 2026 13:19:34 -0500
Subject: [PATCH] discriminator option: Add default value

`${{ runner.os }}-${{ runner.arch }}'

Closes #33
---
 action.yml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/action.yml b/action.yml
index f699c49..6d10d76 100644
--- a/action.yml
+++ b/action.yml
@@ -49,6 +49,7 @@ inputs:
     description: "Run commands as `$shell $file` (with $file containing the build commands)"
   discriminator:
     description: "Unique string to include in the GitHub SDL cache hash"
+    default: ${{ runner.os }}-${{ runner.arch }}
   add-to-environment:
     description: "Add path of the SDL (shared) library to PATH/LD_LIBRARY_PATH/DYLD_LIBRARY_PATH"
     default: "false"