Bazelisk broken with Bazel@Head on Windows

Logs:
https://buildkite.com/bazel/bazel-at-head-plus-downstream/builds/1915#5b5a924a-0e06-4ece-b13d-18161f44f6e2

Error message:
```
# test_bazel_last_green
2021/02/16 04:03:48 Using unreleased version at commit 968422dd6e6735b6066ccbe862b6bc6ab38ce4d6
2021/02/16 04:03:48 Downloading https://storage.googleapis.com/bazel-builds/artifacts/windows/968422dd6e6735b6066ccbe862b6bc6ab38ce4d6/bazel...
Bazelisk version: {STABLE_VERSION}
$TEST_TMPDIR defined: output root default is 'c:\b\cm27gswb\execroot\__main__\_tmp\ab7edd0c907fc9425929721be19c9e77' and max_idle_secs default is '15'.
Extracting Bazel installation...
FATAL: Could not find system javabase. Ensure JAVA_HOME is set, or javac is on your PATH.
```