infra/pkg-download: get rid of the FLOCK variable

infra/pkg-download: get rid of the FLOCK variable

The FLOCK variable is context-dependent, and expects the PKG
variable to be set to the current package.

This is not so nice. Besides, it is used in a single location.

Get rid of this intermediate variable, and directly use flock
where we need it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>