Disable pip cache

We don't really need to keep pip cache as we don't install more
packages with pip after the initial build.
It should reduce the size of the final image of about 50MB.

Signed-off-by: Riccardo Pittau <elfosardo@gmail.com>

prepare-image.sh

@@ -33,7 +33,7 @@ if [[ "$INSTALL_TYPE" == "source" ]]; then
     # NOTE(dtantsur): pip is a requirement of python3 in CentOS
     # shellcheck disable=SC2086
     dnf install -y python3-pip $BUILD_DEPS
-    python3 -m pip install pip==24.1
+    python3 -m pip install --no-cache-dir pip==24.1
 
     IRONIC_PKG_LIST_FINAL="/tmp/ironic-${INSTALL_TYPE}-list-final"
 
@@ -56,7 +56,7 @@ if [[ "$INSTALL_TYPE" == "source" ]]; then
         sed -i '/^ironic-lib===/d' "$UPPER_CONSTRAINTS_PATH"
     fi
 
-    python3 -m pip install --ignore-installed --prefix /usr -r "$IRONIC_PKG_LIST_FINAL" -c "${UPPER_CONSTRAINTS_PATH}"
+    python3 -m pip install --no-cache-dir --ignore-installed --prefix /usr -r "$IRONIC_PKG_LIST_FINAL" -c "${UPPER_CONSTRAINTS_PATH}"
 
     # ironic system configuration
     mkdir -p /var/log/ironic /var/lib/ironic