The userid used during the build is embedded in a shipped tarball in the
mm-common package. Some abbreviated diffoscope output from guix
challenge against builds from ci.guix.gnu.org and bordeax.guix.gnu.org:

│ │ │ │   --- /tmp/guix-directory.rKX8CR/share/doc/mm-common/skeletonmm.tar.xz
│ │ │ ├── +++ /tmp/guix-directory.rlW2tI/share/doc/mm-common/skeletonmm.tar.xz
│ │ │ │ ├── skeletonmm.tar
│ │ │ │ │ ├── file list
│ │ │ │ │ │ @@ -1,36 +1,36 @@
│ │ │ │ │ │ +-rw-r--r--   0 nixbld     (996) nixbld   (30000)       60 2021-05-20 08:57:07.009229 skeletonmm/.gitignore
│ │ │ │ │ │ +-rw-r--r--   0 nixbld     (996) nixbld   (30000)       59 2021-05-20 08:57:07.009229 skeletonmm/AUTHORS
│ │ │ │ │ │ +-rw-r--r--   0 nixbld     (996) nixbld   (30000)    26527 2021-05-20 08:57:07.009229 skeletonmm/COPYING
...
│ │ │ │ │ │ --rw-r--r--   0 nixbld     (995) nixbld   (30000)       60 2021-05-20 08:57:07.009229 skeletonmm/.gitignore
│ │ │ │ │ │ --rw-r--r--   0 nixbld     (995) nixbld   (30000)       59 2021-05-20 08:57:07.009229 skeletonmm/AUTHORS
│ │ │ │ │ │ --rw-r--r--   0 nixbld     (995) nixbld   (30000)    26527 2021-05-20 08:57:07.009229 skeletonmm/COPYING


The attached patch fixes this by setting the user, group, uid and gid
consistently.

  $ guix refresh --list-dependent mm-common
  Building the following 1138 packages would ensure 2236 dependent
  packages are rebuilt: ...

Looks like it will have to wait for core-updates at least...

live well,
  vagrant