We previously explicitly set the HOSTNAME environment variable so that
${HOSTNAME} could be used a placeholder for defining the node.name in
elasticsearch.yml. We removed explicitly setting this because bash
defines HOSTNAME. The problem is that bash defines HOSTNAME as a bash
variable, not as an environment variable. Therefore, to restore the
previous behavior, we export the bash value for HOSTNAME as an
environment variable named HOSTNAME. For consistency between Windows and
the Unix-like systems, we also define HOSTNAME with a value equal to the
environment variable COMPUTERNAME on Windows.
Relates #26262
4e97be02a9