ci: Fix vcvarsall.ps1 environment variable lookup

Previously we were relying on `VCVARSPLATFORM` and `VCVARSVERSION`
powershell script variables.  Read them from the environment directly.

.gitlab/ci/vcvarsall.ps1

@@ -1,6 +1,6 @@
 $erroractionpreference = "stop"
 
-cmd /c "`"$env:VCVARSALL`" $VCVARSPLATFORM -vcvars_ver=$VCVARSVERSION & set" |
+cmd /c "`"$env:VCVARSALL`" $env:VCVARSPLATFORM -vcvars_ver=$env:VCVARSVERSION & set" |
 foreach {
     if ($_ -match "=") {
         $v = $_.split("=")