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
diff --git a/bionic-amd64-tf-a-build/Dockerfile b/bionic-amd64-tf-a-build/Dockerfile
index dc6a24c..ed5cfbd 100644
--- a/bionic-amd64-tf-a-build/Dockerfile
+++ b/bionic-amd64-tf-a-build/Dockerfile
@@ -42,6 +42,7 @@
     python3-dev \
     python3-psutil \
     python3-pyasn1 \
+    python3-venv \
     srecord \
     sudo \
     tree \