Xrootd: Change from xrootd to xrdcp to find a Xrootd executable
xrootd 5.5.4 which is shipped with the current dev4 nightlies and future LCG104 does not contain a xrootd
executable anymore, so the package was not be correctly found anymore in Build/AtlasLCG/modules/FindXrootd.cmake
and lead to ctest failures in the TileCalorimeter/TileExample/TileRecEx
package that uses xrdfs
Changed to find xrdcp
instead now.
Related to ATLASRECTS-7688 - tag @fwinkl who found the culprit/solution.