Use agent 0.2.6 (#2139)

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index ae36613..c7e78c7 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1609,7 +1609,7 @@
     repo = "bazelbuild/continuous-integration"
     if THIS_IS_TESTING and "BAZELCI_AGENT_REPO" in os.environ:
         repo = os.environ["BAZELCI_AGENT_REPO"]
-    version = "0.2.5"
+    version = "0.2.6"
     if THIS_IS_TESTING and "BAZELCI_AGENT_VERSION" in os.environ:
         version = os.environ["BAZELCI_AGENT_VERSION"]
     postfix = ""