Read build and runtime java from properties file (#48355)

This PR changes the PS1 script that starts os tests for the packaging
test matrix to match the bash script we use on Linux in terms of reading
the runtime and build java versions.

Relates to elastic/infra#11593

Author: alpar-t

.ci/os.ps1

@@ -5,10 +5,9 @@ If (-NOT ([Security.Principal.WindowsPrincipal] [Security.Principal.WindowsIdent
     exit
 }
 
-# CI configures these, uncoment if running manually 
-#
-# $env:ES_BUILD_JAVA="java12" 
-#$env:ES_RUNTIME_JAVA="java11" 
+$AppProps = ConvertFrom-StringData (Get-Content .ci/java-versions.properties -raw) 
+$env:ES_BUILD_JAVA=$AppProps.ES_BUILD_JAVA
+$env:ES_RUNTIME_JAVA=$AppProps.ES_RUNTIME_JAVA
 
 $ErrorActionPreference="Stop"
 $gradleInit = "C:\Users\$env:username\.gradle\init.d\"