Set CABAL_DIR and make sure config is adjusted

The config adjustment usually includes adding msys2 directories,
so cabal can find `pkg-config` and libraries, e.g.:

+ C: \ghcup\msys64\mingw64\bin
+ extra-include-dirs: C:\ghcup\msys64\mingw64\include
+ extra-lib-dirs: C:\ghcup\msys64\mingw64\lib
- extra-prog-path: C:\cabal\bin
+ extra-prog-path: C:\ghcup\bin,

images/win/scripts/Installers/Install-Haskell.ps1

@@ -37,7 +37,10 @@ Invoke-PesterTests -TestFile 'Haskell'
 # install minimal ghcup, utilizing pre-installed msys2 at C:\msys64
 $msysPath = "C:\msys64"
 $ghcupPrefix = "C:\"
-Invoke-Command -ScriptBlock ([ScriptBlock]::Create($bootstrapHaskell)) -ArgumentList $false, $true, $true, $false, $false, $false, $false, $ghcupPrefix, "", $msysPath, C:\cabal
+$appdata = [Environment]::GetEnvironmentVariable('APPDATA', [System.EnvironmentVariableTarget]::Machine) 
+$cabalDir = "$appdata\cabal"
+Invoke-Command -ScriptBlock ([ScriptBlock]::Create($bootstrapHaskell)) -ArgumentList $false, $true, $true, $false, $false, $false, $false, $ghcupPrefix, "", $msysPath, $cabalDir
 [Environment]::SetEnvironmentVariable("GHCUP_INSTALL_BASE_PREFIX", $ghcupPrefix, [System.EnvironmentVariableTarget]::Machine)
 [Environment]::SetEnvironmentVariable("GHCUP_MSYS2", $msysPath, [System.EnvironmentVariableTarget]::Machine)
+[Environment]::SetEnvironmentVariable("CABAL_DIR", $cabalDir, [System.EnvironmentVariableTarget]::Machine)
 Add-MachinePathItem "$ghcupPrefix\ghcup\bin"

images/win/scripts/Tests/Haskell.Tests.ps1

@@ -49,6 +49,11 @@ Describe "Haskell" {
         "cabal --version" | Should -ReturnZeroExitCode
     }
 
+    It "cabal config was modified and exists" {
+        [Environment]::GetEnvironmentVariable('CABAL_DIR', [System.EnvironmentVariableTarget]::Machine) | Should -Exist
+        "cabal user-config diff" | Should -Not -BeNullOrEmpty
+    }
+
     It "ghcup is installed" {
         "ghcup --version" | Should -ReturnZeroExitCode
     }