Lorri incorrectly prepend NODE_PATH instead of overwriting

[MichaelBergquistSuarez]

Describe the bug
Lorri sets NODE_PATH to new/path:old/path instead of new/path

To Reproduce
Steps to reproduce the behavior:

1. Have NODE_PATH already set
2. Enter environment
3. echo $NODE_PATH

Expected behavior
NODE_PATH should be overwritten, not prepended

$ lorri info
lorri version: 1.2
GC roots exist, shell_gc_root: "/Users/vimicber/Library/Caches/com.github.target.lorri.lorri.lorri/gc_roots/e92c03aa6865fd95e564539a04da7a12/gc_root/shell_gc_root"

$ uname -a
Darwin l-2016-210.local 19.6.0 Darwin Kernel Version 19.6.0: Tue Jan 12 22:13:05 PST 2021; root:xnu-6153.141.16~1/RELEASE_X86_64 x86_64 i386 MacBookPro11,4 Darwin