Added DRONE_BRANCH to env

[mlnchk]

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[mlnchk]

@googlebot I signed it!

[patrickhulce]

thanks very much @mlnchk! shall we add the other DRONE_* environment variables?

DRONE_COMMIT_SHA
DRONE_REPO

in particular, bummer that there's no build URL variable

[mlnchk]

@patrickhulce commit SHA already works (don't know why), and where to use REPO I have not found. But just in case, I'll add them, thanks :)

[mlnchk]

build url does not exist because Drone is a self-hosted solution I think :(

[patrickhulce]

Thanks @mlnchk !