Write release checksums to per-release files

[aarongable]

Have the release action write the checksums to a file whose name matches the name of the boulder deb and rpm files whose checksums it contains. This prevents name-collision errors when we are running multiple Go versions side-by-side in CI.

To demonstrate this, augment the try-release action to write the checksums to the same file, print the list of files it would upload, and print the contents of the checksums file. Also allow the try-release workflow to be triggered manually.