rustup: rewrite to protect against truncation

This closes #19168. It's possible that if the downloading of `rustup.sh`
is interrupted, bad things could happen, such as running a naked
"rm -rf /" instead of "rm -rf /path/to/tmpdir". This wraps rustup.sh's
functionality in a function that gets called at the last time that should
protect us from these truncation errors.

src/etc/rustup.sh

@@ -455,6 +455,13 @@ install_package() {
     fi
 }
 
+# It's possible that curl could be interrupted partway though downloading
+# `rustup.sh`, truncating the file. This could be especially bad if we were in
+# the middle of a line that would run "rm -rf ". To protect against this, we
+# wrap up the `rustup.sh` destructive functionality in this helper function,
+# which we call as the last thing we do. This means we will not do anything
+# unless we have the entire file downloaded.
+install_packages() {
     rm -Rf "${CFG_TMP_DIR}"
     need_ok "failed to remove temporary installation directory"
 
@@ -479,3 +486,6 @@ fi
 
     rm -Rf "${CFG_TMP_DIR}"
     need_ok "couldn't rm temporary installation directory"
+}
+
+install_packages