Load GPG key from URL rather than keyserver

[Daniel15]

Summary
Loads GPG key from a URL we host, rather than a keyserver. This will hopefully make builds less flaky (they won't fail when the keyserver has issues)

I tried https but it didn't work:

gpgkeys: protocol 'https' not supported
gpg: no handler for keyserver scheme 'https'
gpg: WARNING: unable to fetch URI https://dl.yarnpkg.com/debian/pubkey.gpg: keyserver error

Test plan

Tested the command locally. Will check CircleCI build.

[Daniel15]

CircleCI failure is unrelated - It's a flaky unit test. Going to merge this since adding the GPG key works fine.