Update CHANGELOG.md

CHANGELOG.md

@@ -6,6 +6,7 @@ IMPROVEMENTS:
 * core: https URLs are now supported in the HTTP_PROXY environment variable [go1.10:net/http](https://golang.org/doc/go1.10#net/http)
 * connection/ssh: Add support for host key verifiation [GH-17354]
 * backend/s3: add support for the cn-northwest-1 region [GH-17216]
+* provisioner/local-exec: Allow setting custom environment variables when running commands [GH-13880]
 * provisioner/habitat: Detect if hab user exists and only create if necessary [GH-17195]
 * provisioner/habitat: Allow custom service name [GH-17196]