Auto merge of #53803 - pietroalbini:fix-manifest-2, r=alexcrichton

Fix manifests for broken tools: take 2

This is a follow up of #53715, to avoid stripping unavailable components from the extensions list. This time I also figured out how to test the changes, so the produced manifest is correct.

Along with the fix I added a README with instructions on how to test the tool, and a new `BUILD_MANIFEST_DISABLE_SIGNING` env var to avoid dealing with gpg while testing the tool. I chose an env var instead of a flag because it's more difficult to have it slip in by accident on CI, and there is also another protection that panics if that env var is set on CI, just to be sure we don't release unsigned artifacts.

r? @alexcrichton
cc https://github.com/rust-lang-nursery/rustup.rs/issues/1486