bug#17591: Offload hook fails to send files

[Ludovic Courtès]

This also manifests like this:

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$ ./pre-inst-env guix build hello --no-substitutes
The following derivations will be built:
   /gnu/store/xi7lgm7vyhzhl1z5dy1mba1yki27xkbm-hello-2.9.drv
   /gnu/store/xan82azzisn7b69c0y9856wz2diw3ry1-grep-2.18.drv
hello from "/home/ludo/soft/etc/guix/machines.scm" (PID 1918)
process 1918 acquired build slot '/var/guix/offload/hydra.gnunet.org/0'
load on machine 'hydra.gnunet.org' is 0.2 (normalized: 0.06666666666666667)
@ build-started /gnu/store/xan82azzisn7b69c0y9856wz2diw3ry1-grep-2.18.drv - 
x86_64-linux 
/var/log/guix/drvs/xa//n82azzisn7b69c0y9856wz2diw3ry1-grep-2.18.drv.bz2
sending 2 store files to 'hydra.gnunet.org'...
offloading '/gnu/store/xan82azzisn7b69c0y9856wz2diw3ry1-grep-2.18.drv' to 
'hydra.gnunet.org'...
@ build-remote /gnu/store/xan82azzisn7b69c0y9856wz2diw3ry1-grep-2.18.drv 
hydra.gnunet.org
guix build: error: open-file: No such file or directory: 
"/gnu/store/xan82azzisn7b69c0y9856wz2diw3ry1-grep-2.18.drv"
derivation '/gnu/store/xan82azzisn7b69c0y9856wz2diw3ry1-grep-2.18.drv' 
offloaded to 'hydra.gnunet.org' failed with exit code 1
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Ludo’.