pipeline {
 // snip //
 environment {
  VARIABLES = credentials('MOZILLIANS_VARIABLES')
  SAUCELABS_API_KEY = credentials('SAUCELABS_API_KEY')
  PYTEST_ADDOPTS = "--color=yes --driver=SauceLabs --variables=capabilities.json --variables=${VARIABLES}"
 }
 // snip //
}

WorkflowScript: 25: ""--variables" is not a valid identifier and cannot be used for an environment variable. Identifiers must start with a letter or underscore and can contain only letters, numbers or underscores. @ line 25, column 7.
 "--variables=${VARIABLES}"

pipeline {
 // snip //
 environment {
  VARIABLES = credentials('MOZILLIANS_VARIABLES')
  SAUCELABS_API_KEY = credentials('SAUCELABS_API_KEY')
 }
 stages {
  stage('Test') {
   environment {
    PYTEST_ADDOPTS = "--color=yes --driver=SauceLabs --variables=capabilities.json --variables=${VARIABLES}"
   }
  // snip //
}