add DEVENVFLAVORROOT to support customized flavor dir

[chestercheng]

• Related issue: Support customized flavor directory path #31

• Description:
  With this PR you may define the DEVENVFLAVORROOT environment variable to point to the flavors path where you want.

• Example:

┌── .devenv [feat/customized-flavors-dir]
└─❯ export DEVENVFLAVORROOT=$HOME/.flavors
┌── .devenv [feat/customized-flavors-dir]
└─❯ source scripts/init
┌── .devenv [feat/customized-flavors-dir]
└─❯ devenv use issue31 
now using 'issue31'
┌── .devenv [feat/customized-flavors-dir] (issue31)
└─❯ env | grep ^DEVENV
DEVENVFLAVOR=issue31
DEVENVFLAVORROOT=/home/chestercheng/.flavors
DEVENVPATH_BACKUP=/home/chestercheng/.local/bin:/home/chestercheng/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl
DEVENVPREFIX=/home/chestercheng/.flavors/issue31/usr
DEVENVDLROOT=/home/chestercheng/.devenv/var/downloaded
DEVENVLIBRARY_PATH_BACKUP=
DEVENVROOT=/home/chestercheng/.devenv
┌── .devenv [feat/customized-flavors-dir] (issue31)
└─❯ echo $PATH
/home/chestercheng/.flavors/issue31/usr/bin:/home/chestercheng/.local/bin:/home/chestercheng/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl

[yungyuc]

A user may achieve the same thing by making $DEVENVROOT/flavors a symlink. What is the benefit of having the additional environment variable?

The new environment variable does no harm, though.

[tai271828]

A user may achieve the same thing by making $DEVENVROOT/flavors a symlink. What is the benefit of having the additional environment variable?

The new environment variable does no harm, though.

The first benefit comes to my mind: users who want to create several flavors folder may want to use a environment var.