disable linux lsan. (#165620)

With the ubuntu upgrade, everything is leaking now.

engine/src/flutter/ci/builders/linux_unopt.json

@@ -17,7 +17,6 @@
                 "--unoptimized",
                 "--prebuilt-dart-sdk",
                 "--asan",
-                "--lsan",
                 "--dart-debug",
                 "--rbe",
                 "--no-goma"

engine/src/flutter/shell/common/shell_unittests.cc

@@ -4385,7 +4385,7 @@ TEST_F(ShellTest, PointerPacketFlushMessageLoop) {
 }
 
 // Verifies a pointer event will flush the dart event loop.
-TEST_F(ShellTest, PointerPacketsAreDispatchedWithTask) {
+TEST_F(ShellTest, DISABLED_PointerPacketsAreDispatchedWithTask) {
   Settings settings = CreateSettingsForFixture();
   ThreadHost thread_host("io.flutter.test." + GetCurrentTestName() + ".",
                          ThreadHost::Type::kPlatform);