Downgrade gcc version on AppImage build to 7 from 9

As described in issue
https://github.com/OpenVR-Advanced-Settings/OpenVR-AdvancedSettings/issues/266
the gcc-9 libstdc++ version is not included in Ubuntu 18.04 or
derivatives, which leads to a runtime error.

gcc-7 is the oldest version that has c++17 support. If we didn't need
C++17 support then we could use gcc-5, which is included with Ubuntu
14.04.

.circleci/config.yml

@@ -111,7 +111,7 @@ jobs:
                 command: |
                     sudo add-apt-repository ppa:ubuntu-toolchain-r/test
                     sudo apt-get update
-                    sudo apt-get install g++-9 -y
+                    sudo apt-get install g++-7 -y
             - run:
                 name: Add required QT repos
                 command: |