feat: retrieve build-system metadata from build environment

[jeremylong]

Allows the gradle plugin to retrieve the build-system URL from the environment variables present in most CI systems. By default, this feature is disabled. However, for those who would like to include this information in the generated bom they can configure:

cyclonedxBom {
    includeBuildSystem = true
}

This will attempt to retrieve the build job URL from the CI's environment variables for several CI systems (Jenkins, GitHub Actions, Drone, Jenkins, Circle CI, and Travis CI). If not found, the build-system meta-data is not populated. If you are using an different CI system not listed, you can configure the feature to use alternative environment variables and even a pattern of multiple environment variables:

cyclonedxBom {
    includeBuildSystem = true
    buildSystemEnvironmentVariable = 'MY_CUSTOM_CI_URL'
}

Using multiple environment variables can be achieved using the following (note that you must use single quotes to prevent gradle from interpolating the variables):

cyclonedxBom {
    includeBuildSystem = true
    buildSystemEnvironmentVariable = '${SERVER}/build/${BUILD_ID}'
}

[jeremylong]

I did not make this a configurable field, as in my use cases we want to be able to track back to the actual build that produced the SBOM.

[barblin]

great 👍 , just left some comments.

[barblin]

looks good to me

[skhokhlov]

I like this feature. Left few considerations about the implementation.