Externals Update, including Acts, master branch (2020.08.17.)

This updates all projects to atlasexternals-2.0.73. The list of changes wrt. atlasexternals-2.0.72 is (atlasexternals@2.0.72...2.0.73):

• The installation functions will now throw an error for non-existent files (thanks to @fwinkl);
• Updated FindROOT.cmake to work with ROOT 6.22+ (in preparation for LCG_98python3, and to fix the dev4 nightly);
• Updated all projects not to consider Conda environments in their environment setup (to make it easier to use the "Intel tools" on top of the nightlies);
• Updated flake8_atlas to check for copyright statements correctly under certain conditions (thanks to @fwinkl);
• Updated FindSYCL.cmake to use correct "target platform names" for the latest version of the oneAPI compiler;
• Updated AtlasCMake to set some explicit flags for CUDA builds;
• Updated Acts to v0.31.0.

Changes in this MR fix compilation issues with Acts v0.30.00. Plus an updated had to be made in TrigUpgradeTest to account for the change in the behaviour of the installation functions.

/cc @corentin @akraszna