Enable automated nightly releases (#4397)
Enable automated nightly releases (#4397)

This will automatically create a nightly release
of Orbit on Thursday mornings.

The release version number will be autogenerated
based on "git described" based on the latest
version tag (1.*), e.g. "1.86dev-123-a1b2c3-1".

The changelog file will be automatically filled
based on the git changes since the last nightly
release.

Each release will push a tag of the form
"nightly-%MM-%DD-%YYYY".

Test: Run the workflow on fork.
google/orbit,google/orbit,google/orbit,google/orbit