build: invoke protobufjs CLI explicitly in build:json

[BitHighlander]

Problem

npm run build (and prepublishOnly) fails at build:json:

Error: Expected = but found 3
  at parse (.../protocol-buffers-schema/parse.js)
  at exports.parseSchema (.../pbjs/index.js)

package.json declares both protobufjs@^6.8.8 and an unrelated pbjs@^0.0.5. The latter wins the node_modules/.bin/pbjs symlink (-> ../pbjs/cli.js), so bare pbjs invokes the wrong tool, which can't parse the protos.

This is latent until a clean build actually runs — which keepkey-vault now does on every release build (it rebuilds device-protocol's gitignored lib/ from source), so it currently breaks the release pipeline.

Fix

Invoke protobufjs's CLI explicitly: node node_modules/protobufjs/bin/pbjs. One line, no dependency/lockfile change.

Test plan

• npm run build regenerates a valid 137KB lib/proto.json (verified locally)
• proto set unchanged (still includes messages-zcash.proto)

Follow-up (not here): the pbjs@^0.0.5 devDependency is dead weight that shadows protobufjs's bin — worth removing.