Switch Linux releases to rpm.builder

Summary: Publish Linux Watchman releases through the existing rpm.builder target and remove the legacy Packman definition after its final active consumer moves to fbrpm.

Why: Packman is being retired, and fbrpm builds the same package from a target that lives next to the code.

How: the release step calls fbrpm build with no version overrides, and the target sets use_commit_timestamp_for_version_release so version and release come from the commit being built. --override-version and --override-release are parsed by the CLI but dropped before the publish request is built, so threading them through the determinator would read as if the version were pinned while the published RPM still carried the build machine’s clock.

The conveyor job still checks for the determinator’s start-time version, so it does not agree with the published RPM until the diff at the top of this stack moves those checks onto the commit timestamp too.

Reviewed By: genevievehelsel

Differential Revision: D117902837

fbshipit-source-id: 98d655d7269af2656a08678d0f8f1af5d880e6ac