build: switch AppImage from gcc 9 to gcc 10

This is to make `zxing-cpp` build again. The root cause is an issue in
`pybind11`, see:

* https://github.com/pybind/pybind11/releases/tag/v2.13.1
* pybind/pybind11#5201
* pybind/pybind11#5205
* zxing-cpp/zxing-cpp#803

contrib/build-linux/appimage/Dockerfile_ub2004

@@ -52,13 +52,13 @@ RUN echo deb ${UBUNTU_MIRROR} ${UBUNTU_DIST} main restricted universe multiverse
         libfreetype6=2.10.1-2ubuntu0.3 \
         libfontconfig1=2.13.1-2ubuntu3 \
         libssl-dev=1.1.1f-1ubuntu2.22 \
-        gcc-9=9.4.0-1ubuntu1~20.04.2 \
-        g++=4:9.3.0-1ubuntu2 \
+        gcc-10=10.5.0-1ubuntu1~20.04 \
+        g++-10=10.5.0-1ubuntu1~20.04 \
         rustc=1.75.0+dfsg0ubuntu1~bpo0-0ubuntu0.20.04 \
         cargo=1.75.0+dfsg0ubuntu1~bpo0-0ubuntu0.20.04 \
         && \
-    ln -sf /usr/bin/x86_64-linux-gnu-gcc-9 /usr/bin/gcc && \
-    ln -sf /usr/bin/x86_64-linux-gnu-gcc-9 /usr/bin/cc && \
+    update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-10 10 && \
+    update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-10 10 && \
     rm -rf /var/lib/apt/lists/* && \
     apt-get autoremove -y && \
     apt-get clean