v1.2.0

.github/workflows: split release into pre-release-ci and release (man…

…ual) (#164)

Previously, we run the release workflow automatically whenever a v* tag
gets pushed. The existing workflow does not work because the tests can be
flaky. When that happens, there's no way to release a new TF version since
goreleaser will only run once all the acceptance tests pass.

Until the flakiness goes away, we will split the release workflow into
two: pre-release-ci (that runs automatically like before), and release (which
has to be triggered manually after all the acceptance tests are triaged).