fix(ci): serialize noir-projects nargo dep download in Makefile

[AztecBot]

Summary

Fixes the aztec-nr step failure on merge-train/spartan (CI log http://ci.aztec-labs.com/1779205744876764):

Cloning into '/home/aztec-dev/nargo/github.com/noir-lang/poseidon/v0.3.0'...
fatal: unable to access 'https://github.com/noir-lang/poseidon/': Could not resolve host: github.com

The Makefile builds noir-protocol-circuits, mock-protocol-circuits, noir-contracts, and aztec-nr in parallel — each invokes its own bootstrap.sh. Several share the same noir-lang/poseidon@v0.3.0 and noir-lang/sha256@v0.3.0 git deps, and four parallel nargo processes race on the shared ~/nargo/... cache and outbound DNS. The loser surfaces the failure as a git clone error.

noir-projects/bootstrap.sh already has a serial prep step that documents and works around this race ("Use fmt as a trick to download dependencies. Otherwise parallel runs of nargo will trip over each other trying to download dependencies."), but it only runs when noir-projects/bootstrap.sh is invoked. The root Makefile bypasses it.

Fix

• Move prep to top-level in noir-projects/bootstrap.sh so it can be invoked as ./bootstrap.sh prep. build still calls it — no behavior change for the existing path.
• Add a noir-projects-deps Makefile target that runs prep as a serial barrier, and make the four parallel noir-projects targets depend on it.

Full analysis: https://gist.github.com/AztecBot/c5c82cfc1d739abebd228648bdfe34e4

Test plan

• make -nd aztec-nr shows noir-projects-deps is scheduled once, before any of the four parallel sub-builds.
• bash -n noir-projects/bootstrap.sh passes.
• CI on this PR confirms the fix on real infrastructure (local ./bootstrap.sh ci blocked by a container UID mismatch).

ClaudeBox log: https://claudebox.work/s/5c5ca564e2a8aa3d?run=1