Trying James's version

https://www.jamescroft.co.uk/setting-github-actions-environment-variables-in-powershell/

Author: gbakeman

.github/workflows/build-validate.yaml

@@ -40,7 +40,7 @@ jobs:
         Write-Output "::echo::on"
         $calcBuildNum = ([int]$env:GITHUB_RUN_NUMBER + 8303)
         echo "Calculated number $calcBuildNum"
-        "BUILD_NUM=$calcBuildNum" >> $env:GITHUB_ENV
+        echo "BUILD_NUM=$calcBuildNum" | Out-File -FilePath $env:GITHUB_ENV -Encoding utf8 -Append
         echo "Set Build version to $env:BUILD_NUM"
 
     - name: (Debug) Output ENV vars