build: restore Go 1.20 minimum

.github/workflows/go.yml

@@ -12,18 +12,30 @@ on:
 jobs:
 
   build:
+    name: build (Go ${{ matrix.go }})
     runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        # Go 1.20 checks downstream compatibility; stable tracks supported Go.
+        go: ['1.20', 'stable']
+    env:
+      # Use setup-go's selected toolchain without automatic downloads or switches.
+      GOTOOLCHAIN: local
     steps:
     - uses: actions/checkout@v4
 
     - name: Set up Go
-      uses: actions/setup-go@v4
+      uses: actions/setup-go@v5
       with:
-        go-version: '1.23'
+        go-version: ${{ matrix.go }}
 
     - name: Build
       run: go build -v ./...
 
+    - name: Test
+      run: go test ./...
+
   llgo-test:
     name: llgo-test (${{ matrix.os }})
     runs-on: ${{ matrix.os }}

go.mod

@@ -1,3 +1,3 @@
 module github.com/goplus/lib
 
-go 1.23
+go 1.20