[Test] Revert (partially) Fix windows buildkite (#26615) (#27495)

Root cause:
https://www.shell-tips.com/bash/source-dot-command/#gsc.tab=0
Using . will execute the command in the "current shell" in a bash script. It looks like removing . command from ci.sh init means that we will lose the set -eo command used within ci.sh init applied to next test running commands because set -eo is called within a child process, not the current shell (so the future command won't have the set -eo configured).

.buildkite/pipeline.macos.yml

@@ -12,7 +12,7 @@ prelude_commands: &prelude_commands |-
   rm -rf /tmp/bazel_event_logs
   cleanup() { if [ "${BUILDKITE_PULL_REQUEST}" = "false" ]; then ./ci/build/upload_build_info.sh; fi }; trap cleanup EXIT
   (which bazel && bazel clean) || true;
-  ./ci/ci.sh init && source ~/.zshrc
+  . ./ci/ci.sh init && source ~/.zshrc
   ./ci/ci.sh build
   ./ci/env/install-dependencies.sh
   ./ci/env/env_info.sh
@@ -44,7 +44,7 @@ steps:
     - export MAC_WHEELS=1
     - export MAC_JARS=1
     - export RAY_INSTALL_JAVA=1
-    - ./ci/ci.sh init && source ~/.zshrc
+    - . ./ci/ci.sh init && source ~/.zshrc
     - ./ci/ci.sh build
     # Test wheels
     - ./ci/ci.sh test_wheels

.buildkite/pipeline.windows.yml

@@ -31,7 +31,7 @@ steps:
     - *prelude_commands
     # conda init should be moved to the docker image setup
     - conda init
-    - ./ci/ci.sh init
+    - . ./ci/ci.sh init
     - ./ci/ci.sh build
     - if [ "${BUILDKITE_PARALLEL_JOB}" = "0" ]; then ./ci/ci.sh test_core; fi
     # The next command will be sharded into $parallelism shards.
@@ -41,6 +41,6 @@ steps:
   commands:
     - *prelude_commands
     - export WINDOWS_WHEELS="1"
-    - ./ci/ci.sh init
+    - . ./ci/ci.sh init
     - ./ci/ci.sh build
     - *upload_wheels_if_needed