fix(*.opam): Add missing version

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

--8<---------------cut here---------------start------------->8---
$ 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…
--8<---------------cut here---------------end--------------->8---

1 files changed, 2 insertions, 0 deletions

diff --git a/coq.opam b/coq.opam
index 585e9df789..4399dafe7c 100644
--- a/coq.opam
+++ b/coq.opam
@@ -18,6 +18,8 @@ bug-reports: "https://github.com/coq/coq/issues"
 dev-repo: "git+https://github.com/coq/coq.git"
 license: "LGPL-2.1"
 
+version: "dev"
+
 depends: [
   "ocaml"     {         >= "4.05.0" }
   "dune"      { build & >= "1.10.0"  }