build-tf-a.sh: add poetry tool to system path

TF-A build added poetry as mandatory dependency, required to run tools
like `cot-dt2c`. TF-A's poetry environment is installed during the
install pre-requisites stage. However by default, the installation
command installs the tool to "$HOME/.local/bin/". To ensure that the
poetry executable is correctly located at TF-A build time, add
"$HOME/.local/bin/" to the system path in build-tf-a.sh.

Signed-off-by: Gautham Ravichandran <gautham.ravichandran@arm.com>
Change-Id: I092f2551c98778b552c1e289669313fc1daaf499