bug#34030: Offloading sets mtime relative to current timezone

[Ludovic Courtès]

The ‘reset-timestamps’ procedure in (guix store database) apparently
sets the time relative to the current timezone, leading to
discrepancies:

--8<---------------cut here---------------start------------->8---
$ TZ=UTC stat 
/gnu/store/9fn3k60lgg28r40p2b74nzhy5v3a9cp8-guix-dbd346a/./guix/ftp-client.scm 
/gnu/store/c1ndi2hbg1fcs06gwmbwp297ky2pkw7h-guix-0.16.0-8.7ba2b27/lib/guile/2.2/site-ccache/guix/ftp-client.go
  File: 
/gnu/store/9fn3k60lgg28r40p2b74nzhy5v3a9cp8-guix-dbd346a/./guix/ftp-client.scm
  Size: 12062           Blocks: 24         IO Block: 4096   regular file
Device: 811h/2065d      Inode: 169314017   Links: 73
Access: (0444/-r--r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2019-01-10 10:14:11.376891486 +0000
Modify: 1970-01-01 00:00:01.000000000 +0000
Change: 2019-01-10 13:57:56.753652848 +0000
 Birth: -
  File: 
/gnu/store/c1ndi2hbg1fcs06gwmbwp297ky2pkw7h-guix-0.16.0-8.7ba2b27/lib/guile/2.2/site-ccache/guix/ftp-client.go
  Size: 110469          Blocks: 216        IO Block: 4096   regular file
Device: 811h/2065d      Inode: 349340114   Links: 435
Access: (0444/-r--r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
Access: 2019-01-09 17:57:27.717568033 +0000
Modify: 1969-12-31 23:00:01.000000000 +0000
Change: 2019-01-08 17:57:01.368666736 +0000
--8<---------------cut here---------------end--------------->8---

The time should always be one-second-after-Epoch, UTC.

Ludo’.