Read script from HEAD when --script is set (#1860)

We have to append the date as (ignored) query string so that we don't
get an older, cached version of the script.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 5c27d08..040ff3d 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -57,8 +57,8 @@
     BUILDKITE_ORG
 ]
 
-SCRIPT_URL = "https://raw.githubusercontent.com/bazelbuild/continuous-integration/{}/buildkite/bazelci.py?{}".format(
-    GITHUB_BRANCH, int(time.time())
+SCRIPT_URL = "https://raw.githubusercontent.com/bazelbuild/continuous-integration/{}/buildkite/bazelci.py".format(
+    GITHUB_BRANCH
 )
 
 AGGREGATE_INCOMPATIBLE_TEST_RESULT_URL = "https://raw.githubusercontent.com/bazelbuild/continuous-integration/{}/buildkite/aggregate_incompatible_flags_test_result.py?{}".format(
@@ -3101,7 +3101,7 @@
 
 
 def fetch_bazelcipy_command():
-    return "curl -sS {0} -o bazelci.py".format(SCRIPT_URL)
+    return "curl -sS {0}?{1} -o bazelci.py".format(SCRIPT_URL, int(time.time()))
 
 
 def fetch_aggregate_incompatible_flags_test_result_command():