Bug: yarn might not be installed even when yarn.lock exists

[tszngai]

So this if statement might return false even when yarn.lock exists, resulting yarn not being installed:

nodejs-engine-buildpack/bin/build

Line 35 in 08a1523

if [[ -f "yarn.lock" ]]; then

The affected if block:

if [[ -f "yarn.lock" ]]; then
	install_or_reuse_yarn "$layers_dir/yarn" "$build_dir"
	export PATH=$layers_dir/yarn/bin:$PATH
fi

I think it might come from this line which changes the workspace directory:

nodejs-engine-buildpack/lib/bootstrap.sh

Line 54 in 08a1523

cd "$bp_dir"

Should be an easy fix. I propose two approaches:

Option 1: Change if [[ -f "yarn.lock" ]]; then to if [[ -f "$build_dir/yarn.lock" ]]; then
Option 2: Add cd $build_dir after this line:

nodejs-engine-buildpack/bin/build

Line 25 in 08a1523

bootstrap_buildpack "$layers_dir/bootstrap"

Thanks!