Separate build pre-/post-validation into relevant CI sections

appveyor.yml

@@ -59,23 +59,25 @@ install:
 - sh: export PATH="$HOME/.dotnet:$PATH"
 before_build:
 - dotnet --info
-# Touch T4 templates to force code generation & validation
-- touch MoreLinq/*.g.tt
-build_script:
 - pwsh: |
     grep --extended-regexp '^[[:space:]]*using[[:space:]]+System\.Linq;' (dir -Recurse -File -Filter *Test.cs MoreLinq.Test)
     if ($LASTEXITCODE -eq 0) {
         throw 'Unit tests should not import System.Linq'
     } else {
         $LASTEXITCODE = 0
     }
+# Touch T4 templates to force code generation & validation
+- touch MoreLinq/*.g.tt
+build_script:
 - ps: |
     $id = $env:APPVEYOR_REPO_COMMIT_TIMESTAMP -replace '([-:]|\.0+Z)', ''
     $id = $id.Substring(0, 13)
     if ($isWindows) { cmd /c call pack.cmd ci-$id } else { ./pack.sh ci-$id }
     if ($LASTEXITCODE -ne 0) {
         throw "Building/Packing failed with an exit code of $LASTEXITCODE."
     }
+after_build:
+- ps: |
     $diff = git diff --ignore-all-space --exit-code 2>&1
     $diff | % { if ($_ -is [string]) { $_ } else { [string]$_ } } | echo
     if ($LASTEXITCODE -ne 0) {