Externals Update, master branch (2020.10.12.)

Updated all projects to atlasexternals-2.0.81. The full list of changes wrt. atlasexternals-2.0.80 is the following (atlasexternals@2.0.80...2.0.81):

• CheckerGccPlugins updates for thread safety checks in IStateful::initialize and CORAL (thanks to @ssnyder);
• Added Findpygraphviz.cmake to AtlasLCG (thanks to @fwinkl);
• Fixed the RPM dependency setup in FindCairo.cmake;
  • This was discovered in the 21.0 x86_64-centos7-gcc62-opt builds, and was not affecting the master branch. But it's still good to fix it here as well...
• Updated to onnxruntime 1.5.1 (thanks to @dbakshig);
• Updated to Acts 1.2.0 (thanks to @bschlag);
• Multiple tweaks to AtlasCMake and AtlasLCG to make certain non-standard build setups possible;

These should all just be technical changes, not but it's still prudent to use full-build. (However, I don't believe we'll need a CI node wipe when it's merged in.)