Fix existing existing environment variable filter (#717)

This boolean was reversed when filtering existing variables,
so it consistently sent a POST request to existing vars.

Signed-off-by: Kyle Harding <kyle@balena.io>

Author: klutchell

lib/plugins/environments.js

@@ -167,7 +167,7 @@ module.exports = class Environments extends Diffable {
       for (const variable of attrs.variables) {
         const existingVariable = existingVariables.find((_var) => _var.name === variable.name)
         if (existingVariable) {
-          existingVariables = existingVariables.filter(_var => _var.name !== variable.name)
+          existingVariables = existingVariables.filter(_var => _var.name === variable.name)
           if (existingVariable.value !== variable.value) {
             await this.github.request('PATCH /repos/:org/:repo/environments/:environment_name/variables/:variable_name', {
               org: this.repo.owner,