chore: define OPENSSL_STATIC using ENV

Dockerfile

@@ -32,9 +32,10 @@ RUN make install_sw
 WORKDIR ${cryptography_dir}
 ENV PATH="/root/.cargo/bin:${PATH}"
 ENV OPENSSL_DIR=${openssl_dir}
+ENV OPENSSL_STATIC=1
 RUN python3 -m venv venv
 RUN . ${cryptography_dir}/venv/bin/activate && \
-    OPENSSL_STATIC=1 OPENSSL_DIR="${OPENSSL_DIR}" python3 -m pip install cryptography --no-binary cryptography -v
+    python3 -m pip install cryptography --no-binary cryptography -v
 
 # This is the main mitmproxy container that will be run. We use a new image so
 # the build tools are not left over in the final image.