Truncate: do not use O_EXCL on Plan 9

Signed-off-by: Ronald G Minnich <rminnich@gmail.com>

Author: rminnich

file.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"math"
 	"os"
+	"runtime"
 	"time"
 
 	"github.com/go-git/go-billy/v5"
@@ -249,7 +250,11 @@ func (s *SetFileAttributes) Apply(changer billy.Change, fs billy.Filesystem, fil
 		if curr.Mode()&os.ModeSymlink != 0 {
 			return &NFSStatusError{NFSStatusNotSupp, os.ErrInvalid}
 		}
-		fp, err := fs.OpenFile(file, os.O_WRONLY|os.O_EXCL, 0)
+		flags := os.O_WRONLY
+		if runtime.GOOS != "plan9" {
+			flags |= os.O_EXCL
+		}
+		fp, err := fs.OpenFile(file, flags, 0)
 		if errors.Is(err, os.ErrPermission) {
 			return &NFSStatusError{NFSStatusAccess, err}
 		} else if err != nil {