Fix build on Solaris

internal/upgrade/upgrade_windows.go

@@ -19,10 +19,16 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+
+	"bitbucket.org/kardianos/osext"
 )
 
 // Upgrade to the given release, saving the previous binary with a ".old" extension.
-func upgradeTo(path string, rel Release, archExtra string) error {
+func upgradeTo(rel Release, archExtra string) error {
+	path, err := osext.Executable()
+	if err != nil {
+		return err
+	}
 	expectedRelease := fmt.Sprintf("syncthing-%s-%s%s-%s.", runtime.GOOS, runtime.GOARCH, archExtra, rel.Tag)
 	if debug {
 		l.Debugf("expected release asset %q", expectedRelease)