This commit adds checks for a set of predefined environment variables:
- PULUMI_CI_SYSTEM
- PULUMI_CI_BUILD_ID
- PULUMI_CI_BUILD_TYPE
- PULUMI_CI_BUILD_URL
- PULUMI_CI_PULL_REQUEST_SHA
If PULUMI_CI_SYSTEM is set in the environment, CI configuration is
extracted from the remaining variables for sending to the backend, and
disables the checks for supported systems (currently only Travis CI).
This increases the flexibility of the Pulumi CLI by not requiring
specific support for particular CI systems to be added, provided the
necessary environment variables are configured for the job - this should
be possible for at least TeamCity, Jenkins, AWS CodeBuild, Azure DevOps
Pipelines, and likely most other systems.
This should not replace native support for detecting more CI systems in
future, however, since it requires more work of the user.
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