Set binary name for macOS releases

Before this change, macOS archive used `L2-node` as the binary name,
while all other platforms used `saturn-L2-node` instead.

This commit fixes the problem by making macOS consistent with other
platforms.

Signed-off-by: Miroslav Bajtoš <saturn@bajtos.net>

.goreleaser.yaml

@@ -26,6 +26,7 @@ builds:
     goarch:
       - amd64
     main: ./cmd/saturn-l2
+    binary: saturn-L2-node
 archives:
   - builds:
     - saturn