fix(*.opam): Add missing version

"dev" was not implied, and this can cause the following issues:

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$ docker run --rm -it coqorg/base:latest

 coq@…:~$ opam update
 coq@…:~$ git clone https://github.com/coq/coq.git
 Cloning into 'coq'...

 coq@…:~$ cd coq
 coq@…:~/coq$ opam pin add -n -k path coq .
 [coq.8.10.1] synchronised from file:///home/coq/coq
 coq is now pinned to file:///home/coq/coq (version 8.10.1)

  # issue 1. opam picks the first version he finds for coq from repos…

 coq@…:~/coq$ mv coq.opam foo.opam
 coq@…:~/coq$ opam pin add -n -k path foo .
 Package foo does not exist, create as a NEW package? [Y/n] y
 [foo.~dev] synchronised from file:///home/coq/coq
 foo is now pinned to file:///home/coq/coq (version ~dev)

  # issue 2. if none is found, opam sticks to some "~dev" version…
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0 files changed, 0 insertions, 0 deletions