Update build_command_buffer.py to copy libcommand_buffer_gles2.so from the correct dir on linux

Apparently the linux build now puts the library in chromium's output dir rather than a lib/ subdir.

TBR=borenet@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2165013003

Review-Url: https://codereview.chromium.org/2165013003
1 file changed