Sample build in contributing guide doesn't work

[dgageot]

The contributing guide says these commands can be used to compile a buildpack:

export runtime=nodejs
export buildpack=npm
bazel build "cmd/${runtime}/${buildpack}:${buildpack}.tar"

I get this error:

$ bazel build "cmd/${runtime}/${buildpack}:${buildpack}.tar"
ERROR: Skipping 'cmd/nodejs/npm:npm.tar': no such target '//cmd/nodejs/npm:npm.tar': target 'npm.tar' not declared in package 'cmd/nodejs/npm' (did you mean 'npm.tgz'?) defined by /Users/dgageot/src/buildpacks/cmd/nodejs/npm/BUILD.bazel
WARNING: Target pattern parsing failed.
ERROR: no such target '//cmd/nodejs/npm:npm.tar': target 'npm.tar' not declared in package 'cmd/nodejs/npm' (did you mean 'npm.tgz'?) defined by /Users/dgageot/src/buildpacks/cmd/nodejs/npm/BUILD.bazel
INFO: Elapsed time: 0.078s
INFO: 0 processes.
FAILED: Build did NOT complete successfully (0 packages loaded)