Reduce duplicate jenkins buildsteps

[peterfpeterson]

Jenkins was running the steps to download data more than once for jobs with docs and unit tests. This also defers to ninja how many threads to use.

There is no associated issue.

To test:

If the builds pass, this was a reasonable refactor. Otherwise looking at how the main if/else was combined and looking at a linux job logs will help see if the changes are correct.

This does not require release notes because it is a change to the buildserver.

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[MialLewis]

A couple of queries.

Haven't managed to look at jenkins output yet as it's currently operating at a glacial pace.

[MialLewis]

I think it would make sense to combine this into an if/else statement with line 36-39?

In this implementation the fact we don't use ninja on windows is inferred as we disregard line 38 if using ninja. We may as well make this explicit?

[peterfpeterson]

I could add a comment to that effect. I like to keep the platform and the tools as separate things in case the tool becomes more platform generic.

This comes from long ago when pre-processor macros in the code assumed clang==osx which made it really hard when people started testing out clang on linux.