dev/dev-sourcegraph-server.sh fails with 'Command "build" not found'

Created by: ijt

dev/dev-sourcegraph-server.sh runs ./cmd/frontend/pre-build.sh, which fails like this

++ dirname ./cmd/frontend/pre-build.sh
+ cd ./cmd/frontend/../..
+ echo '--- yarn'
--- yarn
+ [[ -z '' ]]
+ yarn
yarn install v1.15.2
[1/5] Validating package.json...
[2/5] Resolving packages...
success Already up-to-date.
$ gulp generate && yarn --cwd shared -s run tslint:build-rules
[04:04:14] Requiring external module ts-node/register
Using mode development
[04:04:15] Using gulpfile ~/go/src/github.com/sourcegraph/sourcegraph/gulpfile.ts
[04:04:15] Starting 'generate'...
[04:04:15] Starting 'schema'...
[04:04:15] Starting 'graphQLTypes'...
[04:04:17] Finished 'schema' after 1.44 s
[04:04:17] Finished 'graphQLTypes' after 1.44 s
[04:04:17] Finished 'generate' after 1.44 s
Done in 9.28s.
+ echo '--- yarn run build'
--- yarn run build
+ NODE_ENV=production
+ DISABLE_TYPECHECKING=true
+ yarn run build
yarn run v1.15.2
error Command "build" not found.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

Apparently it needs to be running within the web subdirectory.