Fix reproducible builds.
Fix reproducible builds.

-trimpath is available as of Go 1.13, and u-root started requiring Go
1.13 a while ago.

Signed-off-by: Chris Koch <cd2178739c1fb8f241acc6c7bd5abe02c150d7e5@google.com>

u-root/gobusybox,u-root/gobusybox