Trigger workflow when pushing to main (#172)

## Context
This PR will enable the workflow when merging a branch to `main`. This
will allow to build caches if needed for the default `main` branch
immediately after merging. And PRs from other branches can restore the
caches from `main` directly instead of building caches for each branch.

## Scope
`simulation_test.yml`

## Review
* [x] PR description is complete
* [ ] Coding style (imports, function length, New functions, classes or
files) are good
* [ ] Unit/Scientific test added
* [ ] Updated Readme, in-code, developer documentation

---------

Co-authored-by: Fernando Pereira <fernando.pereira@epfl.ch>

.github/workflows/simulation_test.yml

@@ -6,6 +6,9 @@ on:
   pull_request:
     branches:
       - main
+  push:
+    branches:
+      - main
   workflow_dispatch:
     inputs:
       NEURON_BRANCH: