* Add a var for PRNumber. Add an environment metadata key for PR number.
* Move the detection of PULUMI_CI_SYSTEM into vars.DetectVars(). Set the PRNumber CI property based on respective env vars from each CI system.
* Add Azure Pipelines build variables.
* Add tests for DetectVars.
* Added changelog entry for Azure Pipelines.
* Capture the value of env var being modified for the ciutil unit test, and restore their values at the end of them.
* Simplify the DetectVars function by moving the Pulumi CI system code into the switch-case expression.
* Rename the Pulumi CI system to Generic CI. Include the GenericCI system in the test case for DetectVars.
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