Several install_deps issues

Please fix these install_deps.sh issues:

(1)
The command line for building blender that install_deps is creating contains the wrong python version 3.7 (here on manjaro, 3.8 is standard):

-D PYTHON_VERSION=3.7

Every time I run install_deps, I have to correct this manually. Can it be fixed?

(2)
It seems as if OpenXR and Alembic are downloaded, unpacked and compiled every time install_deps.sh is run (just today it happened five times in a row)

Building Alembic-1.7.12

Connecting to github.com (github.com)|140.82.118.4|:443… connected.

Unpacking Alembic-1.7.12

make -f CMakeFiles/Makefile2 all