bug#39387: Builds are *not* offloaded when using the "--keep-failed" or -K option.

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Consider the following, where 10.42.0.199 is a machine described in
/etc/guix/machines.scm for offloading:

$ ./pre-inst-env guix build php
The following derivation will be built:
   /gnu/store/dj9qi9lj4xz0dk6agq9mndh8mhi8ifrg-php-7.4.1.drv
process 8241 acquired build slot '/var/guix/offload/10.42.0.199:22/0'
load on machine '10.42.0.199' is 0.04 (normalized: 0.04)
building /gnu/store/dj9qi9lj4xz0dk6agq9mndh8mhi8ifrg-php-7.4.1.drv...
^C

$ ./pre-inst-env guix build php --keep-failed
The following derivation will be built:
   /gnu/store/dj9qi9lj4xz0dk6agq9mndh8mhi8ifrg-php-7.4.1.drv
building /gnu/store/dj9qi9lj4xz0dk6agq9mndh8mhi8ifrg-php-7.4.1.drv...
starting phase `set-SOURCE-DATE-EPOCH'
phase `set-SOURCE-DATE-EPOCH' succeeded after 0.0 seconds
starting phase `set-paths'
environment variable `PATH' set to
`/gnu/store/niqa6dlvw4r2bfbabn65kq1kgdfwcb4b-pkg-config-0.29.2/bin:/gnu/store/qz06d1dz19wkrmibcr1485wqqia9zzhi-bison-3.5.1/bin:/gnu/store/3fzw8qb3cmrdkr46y55njjrfkvxd53lr-intltool-0.51.0/bin:
[...]

In the second case, the build is started on the local machine instead of
being dispatched to the offload machine.  Presumably, this is done so
that the files are available locally; but I'd prefer if it'd offload and
keep the files on the remote.  An alternative would be to copy the remote build directory
locally after failure.