Fixed inconsistent Alpine version in musl x64 QEMU build (3.15 -> 3.20)

Author: jonatansberg

.github/workflows/ci.yml

@@ -123,7 +123,7 @@ jobs:
         include:
           # musl x64 builds
           - target: linux/amd64
-            variant: alpine3.15
+            variant: alpine3.20
             node: 24
     name: ${{ matrix.variant }} (node=${{ matrix.node }}, target=${{ matrix.target }})
     steps: