bin/fetch-cache-version: Add comment

Document why we are not using `set -euo pipefail`

Co-authored-by: John SJ Anderson <[email protected]>

bin/fetch-cache-version

@@ -1,5 +1,9 @@
 #!/bin/bash
 
+# this script intentionally doesn't `set -euo pipefail`
+# because otherwise the `head -n 2` step triggers SIGPIPE
+# causing the script to exit before it is done.
+
 s3_url="${1:?An S3 URL is required as the first argument}"