Prefer heptools/lcg_platform over binary_tag in manifest.xml

Marco Clemencicrequested to merge
fix-manifest-parsing into master

Since LBCORE-1048, the manifest.xml file contains the field lcg_platform which should be used by lb-run to locate the root directory of LCG installations.

Using binary_tag instead of lcg_platform can cause a failure in the lookup (if they differ), but nobody ever noticed because the default value of LCG_releases_root computed at compile time is perfectly fine.

I noticed the inconsistency by chance, when testing lb-run with the new CMake configuration without the BINARY_TAG environment variable.

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