fix: clear discovery manager when spawing a new one

Author: Luca Bianconi

Diff for: arduino/cores/packagemanager/package_manager.go

@@ -93,6 +93,9 @@ func (pmb *Builder) BuildIntoExistingPackageManager(target *PackageManager) {
 	target.tempDir = pmb.tempDir
 	target.packagesCustomGlobalProperties = pmb.packagesCustomGlobalProperties
 	target.profile = pmb.profile
+	if target.discoveryManager != nil {
+		target.discoveryManager.Clear()
+	}
 	target.discoveryManager = pmb.discoveryManager
 	target.userAgent = pmb.userAgent
 }