Split install_location to file-per-architecture (#59404)

Split install_location to file-per-architecture (#59404)

On non-Windows we used install_location file with multiple lines which contained locations per-architecture (and the default).

This changes the host to use separate file for each architecture:
* `install_location_<arch>` for the architecture specific
* `install_location` for the "legacy" file

This is a much easier format for installers to handle (and it also simplifies the apphost code a little bit).

Adapted tests to the new behavior. Removed some tests which don't make sense anymore. Some small test cleanup to deduplicate code.

Changed the test-only env. variable to point to the directory with configuration files instead of to the configuration file itself.