Fix opensuse image build gate

Gate started to fail with:
    /bin/sh: /usr/bin/python3: Permission denied

Related changes in other projects:
    armada: https://review.opendev.org/#/c/676806/
    pegleg: https://review.opendev.org/#/c/676944/

Change-Id: I66d1ea3af18b7007411d5851df48c61d5d429e17

images/airflow/Dockerfile.opensuse_15

@@ -64,8 +64,9 @@ RUN set -ex && \
         python3-pip \
         python3-devel \
         python3-python-dateutil \
-        make \
-    && python3 -m pip install -U pip \
+        make
+
+RUN pip3 install -U pip \
     && zypper clean -a \
     && rm -rf \
         /tmp/* \

images/shipyard/Dockerfile.opensuse_15

@@ -41,6 +41,7 @@ RUN set -ex && \
         ca-certificates \
         curl \
         netcfg \
+        python3 \
         python3-devel \
         python3-setuptools \
     && zypper clean -a \
@@ -69,17 +70,13 @@ COPY ${ctx_base}/shipyard_client /home/shipyard/shipyard_client/
 COPY ${ctx_base}/shipyard_airflow /home/shipyard/shipyard/
 
 # Build
- RUN set -ex \
-    && buildDeps=' \
-      gcc \
-      git-core \
-      libopenssl-devel \
-      make \
-      python3-pip \
-    ' \
+ENV buildDeps="gcc git-core libopenssl-devel make python3-pip"
+RUN set -ex \
     && zypper -q update -y \
-    && zypper --non-interactive install --no-recommends $buildDeps \
-    && python3 -m pip install -U pip \
+    && zypper --non-interactive install --no-recommends $buildDeps
+
+RUN set -ex \
+    && pip3 install -U pip \
     && pip3 install -r /home/shipyard/client_requirements.txt --no-cache-dir \
     && cd /home/shipyard/shipyard_client \
     && python3 setup.py install \