fix: CI should not rely on external resources

> Retrokits-RK002:arm@1.0.6 0 B / ?    0.00%
> Retrokits-RK002:arm@1.0.6 Server responded with: 403 Forbidden
> Error during install: Server responded with: 403 Forbidden
> ESC[90m<<< Run completed (err = exit status 1)ESC[0m
>     core_test.go:832:
>            Error Trace:    /home/runner/work/arduino-cli/arduino-cli/internal/integrationtest/core/core_test.go:832
>            Error:          Received unexpected error:
>                            exit status 1
>            Test:           TestCoreListSortedResults
> --- FAIL: TestCoreListSortedResults (3.76s)

Author: cmaglie

internal/integrationtest/core/core_test.go

@@ -828,7 +828,7 @@ func TestCoreListSortedResults(t *testing.T) {
 	require.NoError(t, err)
 
 	// install some core for testing
-	_, _, err = cli.Run("core", "install", "test:x86@2.0.0", "Retrokits-RK002:arm", "Package:x86", "--additional-urls="+url.String())
+	_, _, err = cli.Run("core", "install", "test:x86@2.0.0", "arduino:samd", "Package:x86", "--additional-urls="+url.String())
 	require.NoError(t, err)
 
 	// list all with additional url specified