Change text highlight color based on build status

[devatherock]

Vela doesn't expose a build status environment variable as of now, drone does. Use DRONE_BUILD_STATUS to choose the color with logic similar to what is present in the official drone-slack plugin

[devatherock]

Vela exposes a build status variable now - VELA_BUILD_STATUS. It has the value as failure when a previous step has failed and running when none of the previous steps have failed