ax(mining): replace banned errors.New with ErrUnsupportedMiner in xmrig.go

The `errors` package is banned; all errors must use the package's own
error constructors. Replace `errors.New("unsupported operating system")`
with `ErrUnsupportedMiner(runtime.GOOS)` which also surfaces the actual
OS name in the error message.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/xmrig.go

@@ -2,7 +2,6 @@ package mining
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"net/http"
 	"os"
@@ -119,7 +118,7 @@ func (m *XMRigMiner) Install() error {
 	case "darwin":
 		url = fmt.Sprintf("https://github.com/xmrig/xmrig/releases/download/%s/xmrig-%s-macos-x64.tar.gz", version, strings.TrimPrefix(version, "v"))
 	default:
-		return errors.New("unsupported operating system")
+		return ErrUnsupportedMiner(runtime.GOOS)
 	}
 
 	if err := m.InstallFromURL(url); err != nil {