install-node: Fix leaking of $HOME.

This fixes a bug where provision was failing since our most recent
upgrade to yarn/nvm/node.

It turns out my original fix was the correct fix, but to the wrong
third-party tool: nvm, not yarn, was the offender.

scripts/lib/install-node

@@ -10,6 +10,11 @@ YARN_BIN="$ZULIP_SRV/zulip-yarn/bin/yarn"
 node_version=8.11.1
 yarn_version=1.5.1
 
+# This is a fix for the fact that nvm uses $HOME to determine which
+# user account's home directory to ~/.config to.  Ideally, we'd have a
+# more systematic fix, like using `sudo -H` everywhere.
+export HOME=/root
+
 current_node_version="none"
 if hash node 2>/dev/null; then
     current_node_version="$(node --version)"

tools/lib/provision.py

@@ -248,7 +248,7 @@ def main(options):
         print("No changes to apt dependencies, so skipping apt operations.")
 
     # Here we install node.
-    run(["sudo", "scripts/lib/install-node"])
+    run(["sudo", "-H", "scripts/lib/install-node"])
 
     # This is a wrapper around `yarn`, which we run last since
     # it can often fail due to network issues beyond our control.