add SKYBRICKS_DIR and make module file more portable using DESI_ROOT

[sbailey]

@geordie666 this PR updates the etc/desitarget.module file to

• be more portable by using $DESI_ROOT instead of /global/cfs/cdirs/desi
• set SKYBRICKS_DIR=$DESI_ROOT/target/skybricks/v3
• switch $DUST_DIR from ...cosmo/data/dust/v0_1 to ...desi/external/dust/v0_1, which at NERSC is the same thing via links, but allows everything to be under $DESI_ROOT for environment portability

You can inspect the environment that would be installed with this at NERSC via:

source /global/cfs/cdirs/desi/software/desi_environment.sh 21.3
module unload desitarget
module load desitarget/modulefile

(the last two lines are the same as a module swap desitarget/modulefile, but allows you to inspect the environment in between to ensure that it really is unsetting CMX_DIR etc. before resetting correctly with the new environment file.

I'd like to tag this as 1.0.1 to get the $SKYBRICKS_DIR update.

[geordie666]

@sbailey: I tried out the 21.3 environment, and all of the directories behave exactly as I would expect. Feel free to merge this and tag 1.0.1 whenever you get a chance.

Don't forget to update the changes docs!