Skip cloning the repo in the first step of Google Bazel Presubmit

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index d319210..37f70ff 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py

@@ -3705,6 +3705,8 @@
 
 
 def get_modified_files(git_commit):
+    if is_trueish(os.environ.get("DISABLE_MODIFIED_FILES_DETECTION", "false")):
+        return []
     fetch_base_branch()
     merge_base_commit = execute_command_and_get_output(
         ["git", "merge-base", git_commit, "FETCH_HEAD"]

diff --git a/buildkite/bazelci_test.py b/buildkite/bazelci_test.py
index d766125..4dc1e4f 100755
--- a/buildkite/bazelci_test.py
+++ b/buildkite/bazelci_test.py

@@ -570,6 +570,13 @@
         with mock.patch.dict(os.environ, {"BUILDKITE_PULL_REQUEST_LABELS": "foo,CI:run,bar"}):
             self.assertTrue(bazelci.has_presubmit_auto_run_label())
 
+    def test_disable_modified_files_detection(self):
+        with mock.patch.dict(
+            os.environ, {"DISABLE_MODIFIED_FILES_DETECTION": "true"}
+        ), mock.patch.object(bazelci, "fetch_base_branch") as fetch_base_branch:
+            self.assertEqual(bazelci.get_modified_files("abc1234"), [])
+            fetch_base_branch.assert_not_called()
+
 class FetchCiScripts(unittest.TestCase):
     def test_curl_download_command(self):
         cmd = bazelci.curl_download_command("https://example.com/foo.py", "foo.py")

diff --git a/buildkite/terraform/bazel/pipeline.yml.tpl b/buildkite/terraform/bazel/pipeline.yml.tpl
index 74702f1..b651729 100644
--- a/buildkite/terraform/bazel/pipeline.yml.tpl
+++ b/buildkite/terraform/bazel/pipeline.yml.tpl

@@ -12,6 +12,8 @@
       ${command}
 %{ endfor ~}
     label: "${try(steps.label, ":pipeline:")}"
+    checkout:
+      skip: ${try(steps.skip_checkout, false)}
     %{~ if try(steps.priority, null) != null ~}
     priority: ${steps.priority}
     %{~ endif ~}

diff --git a/buildkite/terraform/bazel/pipelines_bazel.tf b/buildkite/terraform/bazel/pipelines_bazel.tf
index b4c0a37..c4f04ee 100644
--- a/buildkite/terraform/bazel/pipelines_bazel.tf
+++ b/buildkite/terraform/bazel/pipelines_bazel.tf

@@ -56,12 +56,15 @@
   default_branch = "master"
   steps = templatefile("pipeline.yml.tpl", {
     envs = {
-      ENABLE_METRICS_COLLECTION = "true"
+      DISABLE_MODIFIED_FILES_DETECTION = "true"
+      ENABLE_METRICS_COLLECTION        = "true"
     },
     steps = {
+      skip_checkout = true,
       commands = [
         "curl -sS \"https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?$(date +%s)\" -o bazelci.py",
-        "bash -c 'set -euo pipefail; python3 bazelci.py project_pipeline --print_shard_summary --file_config=.bazelci/presubmit.yml | tee /dev/tty | buildkite-agent pipeline upload'"
+        "curl -sS \"https://bazel.googlesource.com/bazel/+/$BUILDKITE_COMMIT/.bazelci/presubmit.yml?format=TEXT\" | base64 -d > presubmit.yml",
+        "bash -c 'set -euo pipefail; python3 bazelci.py project_pipeline --print_shard_summary --file_config=presubmit.yml | tee /dev/tty | buildkite-agent pipeline upload'"
       ],
       priority = 99
     }