Add a prompt eval for getting the build target

This is a modified version of a test from the doc. There is a concern
that adding this test will cause the model to get trained on it and
defeat the purpose of the test so I've changed the file and target. It
might be interesting to see if this specific target/file becomes more
reliable than other files/targets. I can also abandon this if there's
any concern about adding a test too similar to the ones being collected

Bug: 443097039
Change-Id: I774d2b41664204565c19c1867f52f4deb2f68f31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6918154
Reviewed-by: Dominic Battré <battre@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Struan Shrimpton <sshrimp@google.com>
Cr-Commit-Position: refs/heads/main@{#1515755}