Avoid unnecessarily setting env variables on the lint build

The relevent env variables are set for the matrix builds, and are irrelevant
to the lint build. By default the first matrix entry is applied.

"Each job included in jobs.include inherits the first value of the array that defines a matrix dimension."
https://docs.travis-ci.com/user/build-stages/#Build-Stages-and-Build-Matrix-Expansion

.travis.yml

@@ -81,6 +81,7 @@ after_script:
 jobs:
   include:
     - stage: lint
+      env:
       sudo: false
       cache: false
       addons: