script: nexantic -> monogon

This replaces the 'nexantic' build container with one called 'monogon'.

Also drive-by fix Fietsje's proto package path.

Test Plan: Refactor, CI only.

X-Origin-Diff: phab/D687
GitOrigin-RevId: 15165f3b57927836dc9601454b31dce4b2c09c7d
diff --git a/scripts/run_in_container.sh b/scripts/run_in_container.sh
index 0fd2731..54c28cf 100755
--- a/scripts/run_in_container.sh
+++ b/scripts/run_in_container.sh
@@ -1,4 +1,4 @@
 #!/bin/bash
 set -euo pipefail
 
-podman exec -it nexantic-dev $@
+podman exec -it monogon-dev $@