Keybase shell command substitutions should use $() instead of backticks

Keybase "command line with [bash + GPG + cURL]" actions produce shell scripts that use backticks for command substitution. Using `$(command)` for command substitutions is preferred over ``command``. See http://mywiki.wooledge.org/BashFAQ/082.

[Backtick command substitutions also produce warnings in shellcheck](https://github.com/koalaman/shellcheck/wiki/SC2006).

Start by locating the Keybase command-line actions that generate the bash + GPG + cURL shell scripts, since the issue names no files or tests. Inspect the generated command substitutions and verify that replacing backticks removes ShellCheck SC2006 warnings without changing script behavior.

Written by the indexing model from the issue text.