Skip flaky retry upgrade test. (#3156)

This will fix the build on main and avoid wasting time while we
investigate the root cause.

See #3155

This test fails often enough for me that we just need to disable it
while we investigate.

testing/integration/upgrade_test.go

@@ -367,6 +367,8 @@ func TestStandaloneUpgradeRetryDownload(t *testing.T) {
 		}, // modifying /etc/hosts
 	})
 
+	t.Skip("Flaky test: https://github.com/elastic/elastic-agent/issues/3155")
+
 	upgradeFromVersion, err := version.ParseVersion(define.Version())
 	require.NoError(t, err)