bug: fix the missing log.Lmsgprefix in go1.13 (#275)

Fixes #274
---------

Co-authored-by: Andy Pan <panjf2000@gmail.com>

.github/workflows/test.yml

@@ -59,7 +59,7 @@ jobs:
       - name: Setup Go
         uses: actions/setup-go@v3
         with:
-          go-version: '^1.13'
+          go-version: ${{ matrix.go }}
 
       - name: Print Go environment
         id: go-env

ants.go

@@ -82,7 +82,9 @@ var (
 		return 1
 	}()
 
-	defaultLogger = Logger(log.New(os.Stderr, "[ants]: ", log.LstdFlags|log.Lmsgprefix|log.Lmicroseconds))
+	// log.Lmsgprefix is not available in go1.13, just make an identical value for it.
+	logLmsgprefix = 64
+	defaultLogger = Logger(log.New(os.Stderr, "[ants]: ", log.LstdFlags|logLmsgprefix|log.Lmicroseconds))
 
 	// Init an instance pool when importing ants.
 	defaultAntsPool, _ = NewPool(DefaultAntsPoolSize)