Allows change INSTALL_DIR and install without sudo

[tobiasrdm]

This feature is useful for installing xq inside pipeline Jobs that do not have sudo permissions.

Example installing to current directory:

curl -sSL https://bit.ly/install-xq | INSTALL_DIR=$(pwd) bash

Example installing to $HOME/.local/bin:

curl -sSL https://bit.ly/install-xq | INSTALL_DIR=$HOME/.local/bin bash

The default behavior remains to install in /usr/local/bin/, which requires sudo.
Example:

curl -sSL https://bit.ly/install-xq | sudo bash