Fix export builds (#486)

* Fix linux debug builds

Amusingly enough godot doesn't actually build the C# project when
exporting the build - we have to do that ourselves.

This required making all the `.csproj` files have the same .net target
version and then just adding the `dotnet build` command. This will also
probably fix the other platforms, but I can only easily test linux.

This also explains why I didn't encounter the issue when running locally
- my command when running locally is `dotnet build && godot
  --rendering-driver opengl3 # run game`, so I'm always rebuilding.

* Add the build command to the other platforms

Tagging the last Godot 3.5 commit for posterity.

Merge pull request #372 from C7-Game/323-Fix-Zombie-Unit-Crash

Merge pull request #367 from C7-Game/241-Remove-Citizens-From-Destroy…

…ed-Cities

Reset test projects to net472, in an attempt to fix the build.

Merge pull request #205 from C7-Game/Carthage-Background

Add a Carthage background

Merge pull request #157 from C7-Game/FixSAVsNotLoading

Fix SAVs not loading

Changed all self projects to net472

Merge pull request #31 from C7-Game/FLCPerformance

FLC Import Performance

Merge pull request #14 from C7-Game/PcxPerformancePolishing

Pcx Performance Polishing