build: mark karma and dev-server tests are flaky

These tests have become flaky.

(cherry picked from commit 12acab8)

Author: alan-agius4

packages/angular_devkit/build_angular/BUILD.bazel

@@ -312,6 +312,7 @@ LARGE_SPECS = {
     "dev-server": {
         "shards": 10,
         "size": "large",
+        "flaky": True,
         "extra_deps": [
             "@npm//@types/http-proxy",
             "@npm//http-proxy",
@@ -323,6 +324,7 @@ LARGE_SPECS = {
     "karma": {
         "shards": 3,
         "size": "large",
+        "flaky": True,
         "extra_deps": [
             "@npm//karma",
             "@npm//karma-chrome-launcher",