Re-enable buildtools on the CI (#2148)

Rollback of #2143

The tests now pass on the CI, also with `.bazelversion` = `last_green`.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 3f9f3b4..469e45c 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -141,7 +141,6 @@
     "Buildtools": {
         "git_repository": "https://github.com/bazelbuild/buildtools.git",
         "pipeline_slug": "buildtools",
-        "disabled_reason": "https://github.com/bazelbuild/buildtools/issues/1297, https://github.com/bazelbuild/buildtools/issues/1300, https://github.com/bazelbuild/buildtools/issues/1314",
     },
     "CLion Plugin": {
         "git_repository": "https://github.com/bazelbuild/intellij.git",