bug#40021: gnu: Add mm-common.

[Danny Milosavljevic]

/gnu/store/cxlagy3f68pg1rxhx101c62jv76iss9k-mm-common-1.0.0/bin/mm-common-prepare
contains references to both cp and ln, without store path.

That means either they won't work or it will have some spooky dependency to
coreutils that is not declared.

I've pushed a variant to guix master as
commit 4e8bdbf853f1f7e01e18f50c95204c0f39e6edba.

Changes are:

* Make sure to refer to coreutils and sed explicitly (non-native input now... 
hmm).

Pushed follow-up moving coreutils and sed to regular inputs to guix master
as commit 74a658857345b564fe0eb8e38c64044b5432289c.

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