Use toybox/etc for test running and Go builds

soong_ui would already put these in $PATH, but they weren't being used
for the ninja/ckati tests, nor the Go builds.

`find` on mac produces different results from ckati, causing the latest
test suite to fail. I'll look into that, but we'd always fall back to
the toybox `find` anyways, so that's what we're concerned about for
compatibility.

Test: prebuilts/build-tools/build-prebuilts.sh on Linux and Mac
Change-Id: Ied491ba515b6ee5e3130e7ef9d96533987bbb317