release: Make fetch happen in GCE images

We want to fetch distfiles, regardless of whether they contain known
vulnerabilities or we're building images for a different version of
FreeBSD.

Approved by: re (cperciva)
Reviewed by: ivy
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53569

(cherry picked from commit 8234c1899b305bcd23323a5870e459028b91bfe4)
(cherry picked from commit 3b01c7da327853e9f3347ff0cae9d075460f5a27)