Add link to "source" vocab

[sroertgen]

Currently there is no link provided in the built vocabularies that links to the source directory. This should be fixed.

e.g. compare bottom left "Source" here: https://test.skohub.io/acka47/testing-skohub-vocabs/heads/master/w3id.org/kim/hcrt/scheme.de.html

But it is missing here: https://sroertgen.github.io/skohub-docker-vocabs-correspondence/ulb.uni-muenster.de/vocab/skos/bible.he.html

[sroertgen]

I guess this is because an environment parameter is missing during the build.