Not all instance types have DMI

https://github.com/compiler-explorer/infra/blob/8e99f144c32c786f0e2b4a8b4432658c33d46c45/init/log-instance-id.service#L10

e.g. searching for this:

Jan 19 13:09:48Z [ip-172-30-5-43](https://my.papertrailapp.com/systems/ip-172-30-5-43/events?focus=1552716935442743362&selected=1552716935442743362) [instance-id](https://my.papertrailapp.com/groups/1243314/events?q=program%3Ainstance-id&focus=1552716935442743362&selected=1552716935442743362) i-0088bda1b0868ad12
Jan 19 13:16:33Z [ip-172-30-1-240](https://my.papertrailapp.com/systems/ip-172-30-1-240/events?focus=1552718635083767809&selected=1552718635083767809) [instance-id](https://my.papertrailapp.com/groups/1243314/events?q=program%3Ainstance-id&focus=1552718635083767809&selected=1552718635083767809) /usr/bin/cat: /sys/devices/virtual/dmi/id/board_asset_tag: No such file or directory
Jan 19 13:46:41Z [ip-172-30-4-158](https://my.papertrailapp.com/systems/ip-172-30-4-158/events?focus=1552726221216555085&selected=1552726221216555085) [instance-id](https://my.papertrailapp.com/groups/1243314/events?q=program%3Ainstance-id&focus=1552726221216555085&selected=1552726221216555085) i-0d9f6fe7c79fe5075
Jan 19 15:28:26Z [ip-172-30-1-40](https://my.papertrailapp.com/systems/ip-172-30-1-40/events?focus=1552751824242229256&selected=1552751824242229256) [instance-id](https://my.papertrailapp.com/groups/1243314/events?q=program%3Ainstance-id&focus=1552751824242229256&selected=1552751824242229256) /usr/bin/cat: /sys/devices/virtual/dmi/id/board_asset_tag: No such file or directory