bug#25430: `guix system reconfigure' fails with "symlink: File exists"

[Arun Isaac]

`guix system reconfigure' fails on my machine with the following
backtrace.

In ice-9/boot-9.scm:
 160: 11 [catch #t #<catch-closure 8cae00> ...]
In unknown file:
   ?: 10 [apply-smob/1 #<catch-closure 8cae00>]
In ice-9/boot-9.scm:
  66: 9 [call-with-prompt prompt0 ...]
In ice-9/eval.scm:
 432: 8 [eval # #]
In ice-9/boot-9.scm:
2404: 7 [save-module-excursion #<procedure 8ea7c0 at ice-9/boot-9.scm:4051:3 
()>]
4056: 6 [#<procedure 8ea7c0 at ice-9/boot-9.scm:4051:3 ()>]
1727: 5 [%start-stack load-stack #<procedure 8fcca0 at ice-9/boot-9.scm:4047:10 
()>]
1732: 4 [#<procedure 8fd6f0 ()>]
In unknown file:
   ?: 3 [primitive-load 
"/gnu/store/gq6rbsz112r5x3dlaii1w8zs8vcyfzf4-dbus-system-services-builder"]
In ice-9/eval.scm:
 432: 2 [eval # ()]
In srfi/srfi-1.scm:
 616: 1 [for-each #<procedure c3ebd0 at ice-9/eval.scm:416:20 (a)> #]
In unknown file:
   ?: 0 [symlink 
"/gnu/store/f4mfsyzqsqccfh22z0md1s1jsk1m79mw-udisks-2.1.8/share/dbus-1/system-services/org.freedesktop.UDisks2.service"
 ...]

ERROR: In procedure symlink:
ERROR: In procedure symlink: File exists
builder for 
`/gnu/store/dywvmn331drm2j6m474vbycdyiljnc4k-dbus-system-services.drv' failed 
with exit code 1
cannot build derivation 
`/gnu/store/4y4v9bdlv4d1r6imamnmy7xcg0r2bcgs-dbus-configuration.drv': 1 
dependencies couldn't be built
cannot build derivation `/gnu/store/w70gl6wvbimy25xjv6w95cqgqm79xksq-etc.drv': 
1 dependencies couldn't be built
cannot build derivation 
`/gnu/store/gk7bj491ds1rb17xz90brad7rdygybw3-system.drv': 1 dependencies 
couldn't be built
guix system: error: build failed: build of 
`/gnu/store/gk7bj491ds1rb17xz90brad7rdygybw3-system.drv' failed

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