TUN-6270: Import gpg keys from environment variables

We now keep the gpg key inputs configurable. This PR imports base64
encoded gpg details into the build environment and uses this information
to sign the linux builds.

cfsetup.yaml

@@ -48,6 +48,7 @@ stretch: &stretch
       - pip3 install pynacl==1.4.0
       - pip3 install pygithub==1.55
       - pip3 install boto3==1.22.9
+      - pip3 install gnupg==2.3.1
     post-cache:
       # build all packages (except macos and FIPS) and move them to /cfsetup/built_artifacts
       - ./build-packages.sh