package/pkg-download.mk: add <pkg>_DL_ENV variable

package/pkg-download.mk: add <pkg>_DL_ENV variable

This variable can be used by package to pass extra environment
variables to the download logic. It will be used for the Go/Cargo
vendoring.

The <pkg>_DL_ENV variable is intentionally not documented: at this
point, it is not meant to be used by packages directly, but only by
package infrastructures.

Suggested-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr: move the two _ENV variables to the same line]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>