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This allows the builder to specify the path of the ninja binary in
case they want to use a specific version, or an alternate
implementation like samurai[0].

Also, when calling ninja, make sure build target comes after any
options.

[0] https://github.com/michaelforney/samurai

Change-Id: I4295baa9e0e04e61ae092a9b86ea6ddc943b3c92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1975453Reviewed-by: Nico Weber <thakis@chromium.org>
Auto-Submit: Michael Forney <mforney@mforney.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#726552}