Cleanup workflow names and triggers

[kendalharland]

Updates a few workflow names and triggers with a more consistent scheme:

• release-swift-toolchain-schedule.yml => schedule-release-swift-toolchain-5_10.yml
  • This workflow only builds version 5.10 of the swift toolchain and can never be extended to build multiple versions simultaneously (see comment in file)
• release-swift-toolchain-binary-sizes.yml => swift-toolchain-binary-sizes.yml
  • This workflow's releases trigger never worked because GitHub prevents jobs from spawning when release artifacts are created with secrets.GITHUB_TOKEN. Instead it is triggered by workflow-run-swift-toolchain-binary-sizes.yml, so drop 'release-' from the file name and workflow name.
• workflow-run-swift-toolchain-binary-sizes.yml
  • Change the title in the GitHub UI from "Trigger Release ..." to "Workflow Run..." for consistency with other workflows.
• pull-request-swift-toolchain-(github|cirun).yml
  • change on.pull_request.files to on.pull_request.paths. The former causes the workflow to run on every PR because files is not a valid key. The latter correctly filters for the set of files in the list and skips running the workflow if no match is found
  • Make sure each workflow runs if it its own file is modified in a PR.

The scheme being followed here is:

• filename = <trigger>-<workflow>.yml (<trigger> is omitted whenever possible if it is pull_request)
• GitHub name = <trigger> - <workflow>.yml