Project file commands don't correctly handle output directories

[erniecohen]

Dafny version

4.9.0

Code to produce this issue

dfyconfig.toml:

includes = ["foo.dfy"] 
excludes = []

[options]
output = "bin/"

foo.dfy:

method {:test} foo() { expect true; }
method main() {}

### Command to run and resulting output

```code
dafny run/build/test dfyconfig.toml

all of these should succeed.

What happened?

dafny run dfyconfig.toml succeeds, but puts output files in the top-level directory, instead of into bin/. build and test get the (long) error below. Changing the dfyconfig.toml option to output = "bin" causes run and build to succeed, but test to fail with the error that it couldn't copy apphost to bin because the destination is a folder rather than a file.

Dafny program verifier finished with 0 verified, 0 errors
Failed to compile C# source code using 'dotnet build /Users/ecohen/Library/CloudStorage/WorkDocsDrive-Documents/projects/verifier/test/bin/.csproj -o /Users/ecohen/Library/CloudStorage/WorkDocsDrive-Documents/projects/verifier/test/bin'. Command output was:
  Determining projects to restore...
/usr/local/share/dotnet/sdk/8.0.303/NuGet.targets(591,5): error MSB4044: The "GetRestoreProjectStyleTask" task was not given a value for the required parameter "MSBuildProjectName". [/Users/ecohen/Library/CloudStorage/WorkDocsDrive-Documents/projects/verifier/test/bin/.csproj]

Build FAILED.

/usr/local/share/dotnet/sdk/8.0.303/NuGet.targets(591,5): error MSB4044: The "GetRestoreProjectStyleTask" task was not given a value for the required parameter "MSBuildProjectName". [/Users/ecohen/Library/CloudStorage/WorkDocsDrive-Documents/projects/verifier/test/bin/.csproj]

What type of operating system are you experiencing the problem on?

Mac