On the top of first-setup.sh there is 

#!/bin/sh

# Takes the same arguments as mtx-update

# you may change this if you want ...
CONTEXTROOT="$PWD/tex"

and some lines from the bottom 
env PATH="$PWD/bin:$CONTEXTROOT/texmf-$platform/bin:$PATH"


On my linux box I see, given that I'm installing under /opt/luatex/standalone-mkiv-test,
$ export|grep PWD
declare -x PWD="/opt/luatex/standalone-mkiv-test"


Perhaps your box doesn't  set PWD correctly ?


--
luigi

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Yes, as you can see:

~/context$ export | grep PWD      
declare -x OLDPWD="/home/pepe"
declare -x PWD="/home/pepe/context"