Fix CI script on platforms using Python <3.9 (#1635)

Example breakage:
https://buildkite.com/bazel/google-bazel-presubmit/builds/66705#0187ed4a-3b83-4bb9-824a-1aaa62c97034
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index b814035..2e7b6b9 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -2217,7 +2217,7 @@
         if not f.startswith(wanted_prefix):
             continue
 
-        tags = f.removeprefix(wanted_prefix).split(",")
+        tags = removeprefix(f, wanted_prefix).split(",")
         include, exclude = partition_list(tags)
 
         # Skip tests tagged as "manual" by default, unless explicitly requested
@@ -2230,6 +2230,16 @@
     return [], ["manual"]
 
 
+def removeprefix(s, prefix):
+    def rp(p):
+        if s.startswith(p):
+            return s[len(p):]
+        return s
+
+    func = getattr(s, "removeprefix", rp)
+    return func(prefix)
+
+
 def filter_unchanged_targets(
     expanded_test_targets, workspace_dir, bazel_binary, diffbase, git_commit
 ):