tf-m: hafnium: set an explicit pip version

fixes the following error:

  File "/tmp/get-pip.py", line 199, in main
    bootstrap(tmpdir=tmpdir)
  File "/tmp/get-pip.py", line 82, in bootstrap
    from pip._internal.cli.main import main as pip_entry_point
  File "/tmp/tmpcP1hEo/pip.zip/pip/_internal/cli/main.py", line 60
    sys.stderr.write(f"ERROR: {exc}")
                                   ^
SyntaxError: invalid syntax

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Change-Id: I4895c8db072a8878325b022d8f9cb5e4d50d00ee
diff --git a/bionic-amd64-hafnium/Dockerfile b/bionic-amd64-hafnium/Dockerfile
index a8c52e4..c4027ad 100644
--- a/bionic-amd64-hafnium/Dockerfile
+++ b/bionic-amd64-hafnium/Dockerfile
@@ -35,7 +35,7 @@
     apt update -q=2 ;\
     apt install -q=2 --yes --no-install-recommends ${PKG_DEPS} ;\
     # Install Python requirements
-    curl -s https://bootstrap.pypa.io/get-pip.py -o /tmp/get-pip.py ;\
+    curl -s https://bootstrap.pypa.io/3.5/get-pip.py -o /tmp/get-pip.py ;\
     python3 /tmp/get-pip.py ;\
     pip3 install --no-cache-dir fdt==0.3.0 ;\
     # Set Python 3 as default
diff --git a/bionic-amd64-tf-m-build/Dockerfile b/bionic-amd64-tf-m-build/Dockerfile
index fd71557..7afc57c 100644
--- a/bionic-amd64-tf-m-build/Dockerfile
+++ b/bionic-amd64-tf-m-build/Dockerfile
@@ -65,7 +65,7 @@
     apt update -q=2 ;\
     apt install -q=2 --yes --no-install-recommends git-lfs ;\
     # Install Python requirements
-    curl -s https://bootstrap.pypa.io/get-pip.py -o /tmp/get-pip.py ;\
+    curl -s https://bootstrap.pypa.io/3.5/get-pip.py -o /tmp/get-pip.py ;\
     python2 /tmp/get-pip.py ;\
     pip2 install --no-cache-dir -r /opt/requirements_python2.txt ;\
     python3 /tmp/get-pip.py ;\