Use shared singularity cache dir on biowulf and frce

[kelly-sovacool]

Related to #52 and possibly #51

If platform is biowulf or frce, use the shared dir for the singularity cache instead of a local dir.

RENEE/renee

Line 914 in 22a5581

cache = os.path.join(outdir, ".singularity")

[kelly-sovacool]

Maybe we need to add a function to copy files from the shared cache to the output dir?

[kelly-sovacool]

See related discussion CCBR/CHAMPAGNE#161 (comment)

The singularity docs state that the cachedir cannot be shared across users. https://docs.sylabs.io/guides/3.6/user-guide/build_env.html#cache-folders

[kelly-sovacool]

It appears we need to set --sif-cache to the proper shared location automatically for biowulf vs FRCE.