Add breakpoints between passes for INTERNAL_CI

These are implemented as dummy jumps with a script that just exits
with success. Since this script will be sourced, rather than called,
this causes the bootstrap process to exit at that point.
The breakpoints are conditional on INTERNAL_CI (we check for "pass1"
because that's the only pass when script-generator runs, but the
resulting effect is that each pass only bootstraps its own part of
the manifest).

steps/jump/break.sh

@@ -0,0 +1 @@
+exit 0