Support concatenation of multiple private keys in the given secret

.github/workflows/demo.yml

@@ -1,7 +1,21 @@
 on: [push, pull_request]
 
 jobs:
-    load_key_demo:
+    single_key_demo:
+        strategy:
+            matrix:
+                os: [ubuntu-latest, macOS-latest]
+        runs-on: ${{ matrix.os }}
+        steps:
+            - uses: actions/checkout@v1
+            - name: Setup key
+              uses: ./
+              with:
+                  ssh-private-key: |
+                    ${{ secrets.DEMO_KEY }}
+                    ${{ secrets.DEMO_KEY_2 }}
+
+    multiple_keys_demo:
         strategy:
             matrix:
                 os: [ubuntu-latest, macOS-latest]
@@ -12,8 +26,6 @@ jobs:
               uses: ./
               with:
                   ssh-private-key: ${{ secrets.DEMO_KEY }}
-            - run: |
-                ssh-add -l
-                echo SSH_AUTH_SOCK is at $SSH_AUTH_SOCK
-
+              
+