lambda-promtail no longer working with basic auth

[slim-bean]

It looks like #5065 accidentally removed the code which sets the basic auth header on requests when the credentials are defined:

This code here:

https://github.com/grafana/loki/pull/5065/files#diff-3e05d69520c09415677a800b675e72ed81465c0813fba9ea973ccd3c4c1528c2L112

Was not added to the new location where we send logs. Not sure if anything else was missed.

[dannykopping]

I believe this PR adds it back.

[cstyan]

Thanks @dannykopping