Pass string input variables and pull request title though env variables

[maxime-rainville]

Use https://docs.github.com/en/actions/learn-github-actions/environment-variables for input strings so that bash variables $myvar can be used instead of string substitution ${{ input.myvar }} which is vulnerable in the similar way to sql injection

Acceptance critreria

• [ ] Validate that it's done systematically
• [ ] Validate that there's a process and/or doc to make sure it stays that way.

Notes

@emteknetnz has already done the grunt work.

[maxime-rainville]

Added relevant AC to https://github.com/silverstripe/github-actions-ci-cd/issues/36