fix!: require NODE_USE_ENV_PROXY for proxy support (#342)

BREAKING CHANGE: Custom proxy handling has been removed. If you use HTTP_PROXY or HTTPS_PROXY, you must now also set NODE_USE_ENV_PROXY=1 on the action step.

tests/snapshots/index.js.md

@@ -82,6 +82,16 @@ Generated by [AVA](https://avajs.dev).
     POST /app/installations/123456/access_tokens␊
     {"repositories":["create-github-app-token"]}`
 
+## main-proxy-requires-native-support.test.js
+
+> stderr
+
+    'A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
+> stdout
+
+    '::error::A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
 ## main-repo-skew.test.js
 
 > stderr
@@ -333,6 +343,16 @@ Generated by [AVA](https://avajs.dev).
     POST /app/installations/123456/access_tokens␊
     {"repositories":["create-github-app-token"],"permissions":{"issues":"write","pull_requests":"read"}}`
 
+## post-proxy-requires-native-support.test.js
+
+> stderr
+
+    'A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
+> stdout
+
+    '::error::A proxy environment variable is set, but Node.js native proxy support is not enabled. Set NODE_USE_ENV_PROXY=1 for this action step.'
+
 ## post-revoke-token-fail-response.test.js
 
 > stderr