Make basePath environment variable controllable

[pascalwhoop]

Description

This PR #2738 allowed us to control the base paths. Great.
But it's only accessible via the config file. Ideally we'd be able to inject this from CI (because we for example generate one dashboard per data release so the base path will be evidence/version/v0.3.0/

What problem would this solve?

better control over the config

How should it work?

export EVIDENCE_VAR_BASE_PATH=some/base/path 
npm run build