Only build the Gazebo simulation projects instead of all of WPILib.
This reverts commit d068fb321f (#3420). The 18.04 docker images now use GCC 8, which was the original reason for updating. 20.04 uses a much more recent libc, so staying with 18.04 enables more Linux platforms to use the binaries.
d068fb321f
Builds have been intermittently failing due to Gradle running out of memory.
