Actually verify tarball against PGP signature

[Footpad]

This addresses #192

With only this change, gpg logs a (slightly) more promising message:

gpg: Signature made Thu 30 Mar 2023 10:28:52 PM UTC using RSA key ID ED3D1561
gpg: Can't check signature: No public key

I understand that in #74 this is the desired result as it is intentional that swiftenv will not download the public keys. It would be nice if swiftenv did have a utility to help make that easier, but I agree it doesn't need to be part of the install command.

If I then get the Swift public keys as described on Swift.org, we see a successful verification:

gpg: Signature made Mon 12 Sep 2022 07:39:56 AM UTC using RSA key ID ED3D1561
gpg: Good signature from "Swift 5.x Release Signing Key <swift-infrastructure@swift.org>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: A62A E125 BBBF BB96 A6E0  42EC 925C C1CC ED3D 1561