Update environment-variables.md

Author: pavel-d

user/environment-variables.md

@@ -206,6 +206,8 @@ to tag the build, or to run post-build deployments.
 - `TRAVIS_JOB_WEB_URL`: URL to the job log.
 - `TRAVIS_OS_NAME`: On multi-OS builds, this value indicates the platform the job is running on.
   Values are currently `linux`, `osx` and `windows` (beta), to be extended in the future.
+- `TRAVIS_CPU_ARCH`: On [multi-arch](https://docs.travis-ci.com/user/multi-cpu-architectures/) builds, this value indicates the CPU architecture the job is running on.
+  Values are currently `amd64`, `arm64`, `ppc64le` and `s390x`.
 - `TRAVIS_OSX_IMAGE`: The `osx_image` value configured in `.travis.yml`. If this is not set in `.travis.yml`,
   it is empty.
 - `TRAVIS_PULL_REQUEST`: The pull request number if the current job is a pull