Distinct Environment Variables in Shared Build Configurations Overwritten by Local .travis.yml Entries

I’m using a shared build configuration with specific environment variables across multiple pipelines. These variables are different from any that might be defined locally in a pipeline’s .travis.yml file. When there are no environment variables defined in a pipeline’s .travis.yml, the variables from the shared configuration work perfectly. However, when .travis.yml includes its own set of different environment variables, it causes those from the shared configuration to be overwritten. How can we prevent this from happening?