Updated to build all branches and PRs

[CodeCutterUK]

Description

The OpenFaas approach is to build on all branches and PRs via GitHub Actions. This PR updates this repo to follow that pattern

Motivation and Context

• I have raised an issue to propose this change (required)

How Has This Been Tested?

https://github.com/openfaas/templates/actions/runs/354389027

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Version change (see: Impact to existing users)

Impact to existing users

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[alexellis]

Approved