Expose ghcup binary to PATH on windows (#4264)

* Expose ghcup binary to PATH on windows

The bootstrap-haskell.ps1 script uses
'[System.EnvironmentVariableTarget]::User' instead of
'[System.EnvironmentVariableTarget]::Machine', so it appears
ghcup env vars and PATH update never make it. Do these manually
for now.

* 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,

* Expose cabal dir to PATH as well

* Use Set-SystemVariable

* Fix tests

* Use hardcoded cabal dir

* Fix bootstrap-haskell

* Move test after installation

* Print ghcup installation message

* Don't adjust cabal.config (to include mingw paths)

This may break setups.

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

@@ -24,6 +24,15 @@ Describe "Haskell" {
         }
     }
 
+    $ghcupEnvExists = @(
+        @{envVar = "GHCUP_INSTALL_BASE_PREFIX"}
+        @{envVar = "GHCUP_MSYS2"}
+    )
+
+    It "<envVar> environment variable exists" -TestCases $ghcupEnvExists {
+        Test-Path env:\$envVar
+    }
+
     It "Accurate 3 versions of GHC are installed" -TestCases @{ghcCount = $ghcCount} {
         $ghcCount | Should -BeExactly 3
     }
@@ -39,4 +48,13 @@ Describe "Haskell" {
     It "Cabal is installed" {
         "cabal --version" | Should -ReturnZeroExitCode
     }
-}
+
+    It "cabal config was modified and exists" {
+        $env:CABAL_DIR | Should -Exist
+        "cabal user-config diff" | Should -ReturnZeroExitCode
+    }
+
+    It "ghcup is installed" {
+        "ghcup --version" | Should -ReturnZeroExitCode
+    }
+}