add rustc to apt prerequisites

This was apparently an automatic install in the past rather than a
requirement.  An apt upgrade on lab1 caused the pacakge to be removed.
Putting it here will mark it as a manual install and not auto remove it
in the future

Bypass-Presubmit-Reason: no presubmits
Change-Id: I96f2a46522142b54d45da4e7f6aefb1455234e64

install-prereqs.sh

diff --git a/install-prereqs.sh b/install-prereqs.sh
index fbdb1a2..28b640a 100755
--- a/install-prereqs.sh
+++ b/install-prereqs.sh
@@ -86,6 +86,7 @@
     python3-pip
     python3-venv
     rsync
+    rustc
     socat
     srecord
     texinfo