Only run yarn build when necessary in tools/pip.sh

The Zuul unittest jobs were timing out which prompted me to take a look
at what might be taking extra time. One thing I noticed is that we're
running the yarn build (which runs react-scripts) multiple times when we
really only need to run it once. The reason for that is our check to see
if zuul web has already built is looking for a file that the builds no
longer produce. Update that check to look for a current file and we'll
save a bit of time in our jobs and when running things locally.

Change-Id: Iae3604fbaf072d53895db850cfc989a832b12b27

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