fix spec file support for Windows plugin discovery (#25903)

Signed-off-by: Michal Wieczorek <wieczorek-michal@wp.pl>

pkg/plugins/discovery_unix.go

@@ -1,3 +1,5 @@
+// +build !windows
+
 package plugins
 
 var specsPaths = []string{"/etc/docker/plugins", "/usr/lib/docker/plugins"}

pkg/plugins/discovery_windows.go

@@ -5,4 +5,4 @@ import (
 	"path/filepath"
 )
 
-var specPaths = []string{filepath.Join(os.Getenv("programdata"), "docker", "plugins")}
+var specsPaths = []string{filepath.Join(os.Getenv("programdata"), "docker", "plugins")}