ci: use GITHUB_TOKEN when preparing FFI

[galargh]

Requires https://github.com/filecoin-project/filecoin-ffi/pull/310

This should help not getting rate limited when getting FFI.

The error that we encountered during the tests suggests that making authenticated calls is the way to go:

++ curl --retry 3 --location https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/tags/5d00bb4365a97890
++ echo '{"message":"API rate limit exceeded for 208.83.5.75. (But here'\''s the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)","documentation_url":"https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}'

[rvagg]

gets used here: https://github.com/filecoin-project/filecoin-ffi/blob/5d00bb4365a97890/install-filcrypto#L6-L8, but the one place it matters is accounted for in https://github.com/filecoin-project/filecoin-ffi/pull/310 which we're not going to be able to pull in until we catch up to the latest version of filecoin-ffi (when that's merged) .. which could be a while; but we'll merge this and just keep the timeouts in mind until we get to that upgrade point.

[codecov-commenter]

Codecov Report

Merging #104 (f572aae) into master (5afc2de) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #104   +/-   ##
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  Coverage    7.56%   7.56%           
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  Files          12      12           
  Lines        1758    1758           
======================================
  Hits          133     133           
  Misses       1621    1621           
  Partials        4       4           

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