fix: load documentation dependencies from `requirements.txt`

This CI job previously used whatever dependencies were provided by the
Jenkins node's Docker image. This change ensures the Python
dependencies are instead loaded from TF-A's PIP requirements file.

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: I752125c090f3b9252be23983fe2c0eaaa326b225
diff --git a/group/tf-l1-check-docs/build-docs:nil b/group/tf-l1-check-docs/build-docs:nil
index 8354760..02d845d 100644
--- a/group/tf-l1-check-docs/build-docs:nil
+++ b/group/tf-l1-check-docs/build-docs:nil
@@ -6,6 +6,9 @@
 #
 
 pre_tf_build() {
+	python3 -m venv .venv && \
+		. .venv/bin/activate && \
+		python3 -m pip install -r "$tf_root/docs/requirements.txt"
+
 	targets="doc" set_tf_build_targets
 }
-