ci: Clear env to run build

We don't need any of the environment variables Jenkins sets, and some of
them (like `GIT_COMMIT`) interfere with building packages.

ci/Jenkinsfile

@@ -11,7 +11,7 @@ pipeline {
     stages {
         stage('Build') {
             steps {
-                sh 'make -C aimee-os CONFIGDIR=${PWD} O=/build'
+                sh 'env -i PATH="${PATH}" make -C aimee-os CONFIGDIR=${PWD} O=/build'
             }
         }
     }