build: add env variable for datadog service

[halfwhole]

Problem

DD_SERVICE is not consistently defined for Go across elastic beanstalk, tracing, logs, and monitoring. It's also defined differently from DD_ENV even though they serve similar purposes, which may be confusing

Solution

Defined DD_SERVICE within ci.yml in a similar manner to DD_ENV, and standardised its naming to go-[gov/edu/health] (we should be able to tell whether it's staging or production from DD_ENV, so will leave that out of the service naming)

Tests

• Tested on staging, the new service naming takes effect for tracing, logs, monitoring - some application metrics (e.g. for RDS and ELB) are still stuck with the gogovsg naming though, so that's probably configured somewhere else, or the changes will only take effect once it's been released to production