scripts/make.fsx: set DOTNET_ROLL_FORWARD for unit tests

This is to fix all CI failures related to .NET versions mismatch,
which were giving this error:

```
dotnet test /home/runner/work/geewallet/geewallet/src/GWallet.Backend.Tests/GWallet.Backend.Tests.fsproj --configuration Debug
  Determining projects to restore...
  All projects are up-to-date for restore.
  GWallet.Backend -> /home/runner/work/geewallet/geewallet/src/GWallet.Backend/bin/Debug/netstandard2.0/GWallet.Backend.dll
  GWallet.Backend.Tests -> /home/runner/work/geewallet/geewallet/src/GWallet.Backend.Tests/bin/Debug/net6.0/GWallet.Backend.Tests.dll
Test run for /home/runner/work/geewallet/geewallet/src/GWallet.Backend.Tests/bin/Debug/net6.0/GWallet.Backend.Tests.dll (.NETCoreApp,Version=v6.0)
VSTest version 18.0.1 (x64)

Starting test execution, please wait...
A total of 1 test files matched the specified pattern.
Testhost process for source(s) '/home/runner/work/geewallet/geewallet/src/GWallet.Backend.Tests/bin/Debug/net6.0/GWallet.Backend.Tests.dll' exited with error: You must install or update .NET to run this application.
App: /home/runner/work/geewallet/geewallet/src/GWallet.Backend.Tests/bin/Debug/net6.0/testhost.dll
Architecture: x64
Framework: 'Microsoft.NETCore.App', version '6.0.0' (x64)
.NET location: /usr/share/dotnet/
The following frameworks were found:
  8.0.6 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  8.0.22 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  9.0.6 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  9.0.11 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
  10.0.0 at [/usr/share/dotnet/shared/Microsoft.NETCore.App]
Learn more:
https://aka.ms/dotnet/app-launch-failed
To install missing framework, download:
https://aka.ms/dotnet-core-applaunch?framework=Microsoft.NETCore.App&framework_version=6.0.0&arch=x64&rid=linux-x64&os=ubuntu.24.04
. Please check the diagnostic logs for more information.

Test Run Aborted.
Error when running 'dotnet test /home/runner/work/geewallet/geewallet/src/GWallet.Backend.Tests/GWallet.Backend.Tests.fsproj --configuration Debug'
Fsdk.Process+ProcessFailed: Exception of type 'Fsdk.Process+ProcessFailed' was thrown.
   at Fsdk.Process.ProcessResult.Unwrap(String errMsg)
   at Fsdk.Process.ProcessResult.UnwrapDefault()
   at <StartupCode$FSI_0007>.$FSI_0007.main@() in /home/runner/work/geewallet/geewallet/scripts/make.fsx:line 443
   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
Stopped due to error
make: *** [Makefile:8: check] Error 1
Error: Process completed with exit code 2.
```

Author: knocte

scripts/make.fsx

@@ -384,6 +384,9 @@ match maybeTarget with
             testProjectName,
             testProjectName + ".fsproj"
         ) |> FileInfo
+
+    // somehow sometimes the binary is seen by NUnit as 6.0, while we already have a .NET8.0 (or newer) runtime
+    Environment.SetEnvironmentVariable("DOTNET_ROLL_FORWARD", "major")
 #else
     // so that we get file names in stack traces
     Environment.SetEnvironmentVariable("MONO_ENV_OPTIONS", "--debug")