feat: Add configurable PIXI_HOME

[chawyehsu]

Address #619 , probably also #541

command

# Unix
export PIXI_HOME=/your/custom/pixi/home
# Windows
$env:PIXI_HOME="D:\dev\pixi"
pixi global install <package>

[ruben-arts]

Thanks this work looks good.

Could you align the install scripts to use the same PIXI_HOME variable. I see we use PIXI_DIR which would be better to change to PIXI_HOME?
And could you mention this in the configuration documentation that its possible to set this variable.

[chawyehsu]

will do.

[chawyehsu]

Ready for reviews.

[ruben-arts]

Thanks for the addition! Looks good to me!