System report should include "ci:<provider>" tags when running in CI/CD environments

[zomglings]

Some of our customers build software that's intended to run in CI/CD pipelines. It is important for them to have a good sense of which CI/CD tools their users are using their software in.

Most of our customers would also like to be able to filter on events fired from CI/CD or filter them out of their analyses.

I propose that, when we generate system tags, we try to detect whether we are running in a CI/CD environment and add the (inferred) name of the CI/CD provider (e.g. GitHub Actions, CircleCI, TravisCI, etc.) as a tag of the form ci:<provider>.

Each CI/CD platform has certain environment variables that it uses to configure internal processes:

1. GitHub Actions
2. Circle CI
3. Travis CI

Note that all of them seem to set CI=true. So at the very least we should use that. We can also use the presence or absence of other environment variables to infer which specific CI provider we are working with.

[zomglings]

We need to define names for each of the CI environments:

1. GitHub Actions: github
2. Circle CI: circle
3. Travis CI: travis

Will keep this list up-to-date until we release this feature.