nim CI broke because of last commit: Error: cannot open file: layout_vars
/cc @yglukhov 

latest commit https://github.com/yglukhov/nimx/commit/f5902ce59f27d1f078894427b670d9a01036947a in nimx seems to have broken nim CI again (eg https://github.com/nim-lang/Nim/pull/13926 + 1 other PR broke, but also a fake one that I did just to double check that it's due to nimx), giving:
/home/vsts/.nimble/pkgs/nimx-0.1/nimx/view.nim(2, 42) Error: cannot open file: layout_vars

(in this PR that just tests nimx:
https://dev.azure.com/timotheecour/timotheecour/_build/results?buildId=1116&view=logs&j=63665ae6-b751-545e-a6c4-24432959da79&t=9b2d96d9-ecd2-5347-871c-a997abe1fc35)

## how to reproduce
```
git clone https://github.com/yglukhov/nimx
cd nimx
git rev-parse HEAD
f5902ce59f27d1f078894427b670d9a01036947a
# use a fresh dir: rm -rf $HOME/.nimble_fake12
# do what testament does
NIMBLE_DIR=$HOME/.nimble_fake12 nimble install nimx
NIMBLE_DIR=$HOME/.nimble_fake12 nim c -r --threads:on test/main.nim
```

/Users/timothee/.nimble_fake12/pkgs/nimx-0.1/nimx/view.nim(2, 42) Error: cannot open file: layout_vars
  import types, context, animation_runner, layout_vars

I'm noticing nimx does not have a git tag, so there's no notion of stable release, which is why things tend to break often.

## proposal
* fix this bug
* **important**: add a git tag after it's working, and keep updating it on regular basis (could be very often, so long it's at a point where it's working)
* after that, `nimble install` in important_packages should test latest stable release, not HEAD (which is likely to break often)

