Making ./build.sh --with... work better

(This is a WIP issue where I am collecting issues that I am running into when trying to use our various `./build.sh --with-....` options on Fedora)

### `--with-sdk` tries to modify a read-only SDK

I tried using `--with-sdk` with a `/usr/lib64/dotnet` (that's where the previously built SDK is installed) and the build blows up:

```
/home/dotnet/dotnet3.1/dotnet-v3.1.102-SDK/tools-local/init-build.proj(35,5): error MSB3021: Unable to copy file "/home/dotnet/dotnet3.1/dotnet-v3.1.102-SDK/tools-local/tasks/Microsoft.DotNet.SourceBuild.Tasks.XPlat/bin/Debug/netstandard2.0/Microsoft.DotNet.SourceBuild.Tasks.XPlat.dll" to "/usr/lib64/dotnet/sdk/3.1.101/SdkResolvers/SourceBuild.MSBuildSdkResolver/SourceBuild.MSBuildSdkResolver.dll". Access to the path '/usr/lib64/dotnet/sdk/3.1.101/SdkResolvers/SourceBuild.MSBuildSdkResolver' is denied.
```

It's trying to mutate the copy of the SDK at `/usr/lib64/dotnet/` which only `root` can write to.

### TODO