feat: use Netlify context as default tag

Author: bahmutov

README.md

@@ -97,7 +97,7 @@ package = "netlify-plugin-cypress"
 
 #### tag
 
-You can give recorded run [tags](https://on.cypress.io/module-api#cypress-run) using a comma-separated string
+You can give recorded run [tags](https://on.cypress.io/module-api#cypress-run) using a comma-separated string. If the tag is not specified, Netlify context will be used (`production`, `deploy-preview` or `branch-deploy`)
 
 ```toml
 [[plugins]]

src/index.js

@@ -179,6 +179,8 @@ module.exports = function cypressPlugin (pluginConfig) {
         group = preBuildInputs.group || 'preBuild'
         if (preBuildInputs.tag) {
           tag = preBuildInputs.tag
+        } else {
+          tag = process.env.CONTEXT
         }
       }