[util] Avoid using which command in verible-format.py
[util] Avoid using which command in verible-format.py

With recent versions of binutils, "which" spits out a warning message.
Use the Python builtin instead.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan