testrunner: Speed up printing --help by 4x

Before:

$> time ./test/testrunner/testrunner.py --help
real 0m4.261s
user 0m1.806s
sys 0m2.476s

After:

$> time ./test/testrunner/testrunner.py --help
real 0m0.988s
user 0m0.417s
sys  0m0.573s

---------
Reach into the build once instead of 4 times to
get its internal build variables.

Test: make test-art-host
Change-Id: Ia1279e762b0cf6e04cd04849257c5564c3afa417