Fix #34926: set SYSTEM_VERSION_COMPAT=0 when installing STP

Presumably Azure Pipelines (temporarily?) had an image which didn't
set SYSTEM_VERSION_COMPAT=1 by default, hence this previously appeared
to work without this.

tools/ci/azure/install_safari.yml

@@ -9,6 +9,7 @@ steps:
 - ${{ if eq(parameters.channel, 'preview') }}:
   - script: |
       set -eux -o pipefail
+      export SYSTEM_VERSION_COMPAT=0
       ./wpt install --channel preview --download-only -d . --rename STP safari browser
       sudo installer -pkg STP.pkg -target LocalSystem
       # Workaround for `sudo safardriver --enable` not working on Catalina: