Add comment about TMPDIR environment variable

website/source/docs/other/environmental-variables.html.md

@@ -39,3 +39,8 @@ each can be found below:
     [checkpoint.hashicorp.com](https://checkpoint.hashicorp.com/) to look for
     new versions of Packer. If you want to disable this for security or privacy
     reasons, you can set this environment variable to `1`.
+
+-   `TMPDIR` - The location of the directory used for temporary files (defaults
+    to /tmp on Linux/Unix). It might be necessary to customize it when working
+    with large files since /tmp is a memory-backed filesystem in some Linux
+    distributions in which case /var/tmp might be preferred.