bionic-amd64-tf-a-build: install `python3-venv`

We need this for our CI jobs which have broken on a change that was
merged recently (working on the Arm internal CI).

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: I03624ff40c953f40647d714474aed45014f4691a
1 file changed