logic error in spack platform detection logic

[cyrush]

right now, if you aren't running on LC (SYS_TYPE env set) or macos (darwin) you will get the error message:

[ERROR: Given path for 'spack_configs_path' does not contain platform directories: ...]

The logic here is wrong:

uberenv/uberenv.py

Line 615 in 92c4cb4

else:

if platform is None, logic should simply skip trying to find the defaults for that platform.

(I hit this in a docker container)