Configure Git to use bearer token auth mechanism

[michaelsauter]

Tekton prepares the creds to be used through basic auth, which fails when the Bitbucket server has basic auth disabled.

Fixes #683

Tasks:

• [x] Updated design documents in docs/design directory or not applicable
• [x] Updated user-facing documentation in docs directory or not applicable
• [x] Ran tests (e.g. make test) or not applicable
• [x] Updated changelog or not applicable