gitserver: faster and cleaner p4-fusion build script

Created by: keegancsmith

Observing builds for p4-fusion on my machine, it seems the time is dominated by building OpenSSL. This commit speed that up by using concurrency and doing less work. Additionally we add justification for why we need a specific OpenSSL.

We make cleanup more robust by doing all work inside of a temporary directory.

We add buildkite sections to make reading the logs clearer.

We also add a mini test that p4-fusion runs correctly. Note in practice we copy this into another docker file so the test is not that robust. However, it is better than nothing.

Test Plan: Run build and test p4-fusion in the image.

docker build -t p4-fusion --target=p4-fusion .
docker run --rm=true -it p4-fusion /bin/sh -il
ldd `which p4-fusion`
p4-fusion --help