fix: install Python dependencies

This change adds a new script - `install_python_deps_tf.sh` - which
creates and loads a clean Python virtual environment and within it
installs TF-A's Python dependencies.

The previous method for doing this (sourcing the environment from
within the pre-build hook) was not working as the environment was going
out of scope after returning from the function.

Change-Id: I1c2e404665853d33212254ad576a8095b5c25aa0
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/script/build_package.sh b/script/build_package.sh
index 10d1df4..a52b3f3 100755
--- a/script/build_package.sh
+++ b/script/build_package.sh
@@ -1387,6 +1387,8 @@
 			source "$plat_utils"
 		fi
 
+		source "$ci_root/script/install_python_deps_tf.sh"
+
 		archive="$build_archive"
 		tf_build_root="$tf_root/build"