Accept ssh-key as a raw PEM key, drop base64 requirement

ssh-key was expected to be base64-encoded and decoded with
`base64 --decode`, which is inconsistent with ssh-upload and
ssh-command (both take the raw PEM directly) and an easy way to end
up with a silently corrupt key file if the stored secret isn't
actually base64. Write the input straight to the key file instead.

action.yml

@@ -38,8 +38,8 @@ runs:
         mkdir -p ~/.ssh
         chmod 700 ~/.ssh
 
-        # 2. Decode the Base64 secret back into a properly formatted file
+        # 2. Write the private key to a file as-is
-        echo "$SSH_KEY" | base64 --decode > ~/.ssh/gitea_key
+        printf '%s\n' "$SSH_KEY" > ~/.ssh/gitea_key
         chmod 600 ~/.ssh/gitea_key
 
         # 3. Configure SSH for the custom port and bypass host key prompt