Merge pull request #944 from rust-lang/docs-fix-dev-desktops-instructions

docs: fix dev-desktops instructions

Author: marcoieni

terragrunt/modules/dev-desktops-gcp/README.md

@@ -5,4 +5,5 @@
 - Add the public IP address of the machine to `ansible/envs/prod/hosts`
 - Download the buildbot private SSH key from 1Password
 - `chmod 600 <path-to-buildbot-west-slave-key.pem>`
+- `export ANSIBLE_PRIVATE_KEY_FILE=<path-to-buildbot-west-slave-key.pem>`
 - `cd ansible && ./apply prod dev-desktop -u ubuntu`