MSI: Use ENV['TD_AGENT_TOPDIR'] to specify install directory

[kenhys]

On windows, /var/... may creates C:/var, so it should be
ENV['TD_AGENT_TOPDIR'] (typically c:/opt/td-agent) prepended.

[kenhys]

After starting service, /var is created under C:/opt/td-agent/var.

[kenhys]

On buster: no ENV['TD_AGENT_TOPDIR'] prepended.

% cat /etc/td-agent/td-agent.conf | head -n 30
####
## Output descriptions:
##

# Treasure Data (http://www.treasure-data.com/) provides cloud based data
# analytics platform, which easily stores and processes data from td-agent.
# FREE plan is also provided.
# @see http://docs.fluentd.org/articles/http-to-td
#
# This section matches events whose tag is td.DATABASE.TABLE
<match td.*.*>
  @type tdlog
  @id output_td
  apikey YOUR_API_KEY

  auto_create_table
  <buffer>
    @type file
    path /var/log/td-agent/buffer/td
  </buffer>

  <secondary>
    @type file
    path /var/log/td-agent/failed_records
  </secondary>
</match>

## match tag=debug.** and dump to console
<match debug.**>
  @type stdout

[cosmo0920]

LGTM! 👍