[otbn/util] Update RISC-V toolchain search to look in default location.
[otbn/util] Update RISC-V toolchain search to look in default location.

Signed-off-by: Jade Philipoom <23b156e860ef6ab36b055884a79031aa7723874a@google.com>

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