feat: download release tarball from named release

[rvagg]

Avoid the need for using the commit hash to find the release.

Fixes: #491

See #491 for the question of whether we want to lock this behaviour in - release assets are only available for tags, not every commit on master as they used to be with CircleCI.

[rvagg]

Running it from this commit:

[download_release_tarball] failed to determine release tag
+ return 1
+ echo '[install-filcrypto/main] building libfilcrypto from local sources (dir = rust)'

Running it from 1.30.0-dev which has no assets:

+ echo '[download_release_tarball] acquiring release @ v1.30.0-dev'
[download_release_tarball] acquiring release @ v1.30.0-dev
++ curl --retry 3 --location https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/tags/v1.30.0-dev
...
[download_release_tarball] failed to download release (tag URL: https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/tags/v1.30.0-dev, response: {
  "url": "https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/174524183",
  "assets_url": "https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/174524183/assets",
...
  "body": "This is a developer release of Filecoin-FFI intended for testing out https://github.com/filecoin-project/filecoin-ffi/pull/480 and the nv24-skeleton.\r\n\r\n## What's Changed\r\n* ci: automate creation of GitHub releases and tags by @galargh in https://github.com/filecoin-project/filecoin-ffi/pull/475\r\n* feat: FFI_PORTABLE to build a maximally portable binary by @rvagg in https://github.com/filecoin-project/filecoin-ffi/pull/480\r\n\r\n\r\n**Full Changelog**: https://github.com/filecoin-project/filecoin-ffi/compare/v1.29.0-dev...v1.30.0-dev",
  "mentions_count": 2
})
+ return 1
+ echo '[install-filcrypto/main] building libfilcrypto from local sources (dir = rust)'

Running it from 1.30.0-dev2:

+ echo '[download_release_tarball] acquiring release @ v1.30.0-dev2'
[download_release_tarball] acquiring release @ v1.30.0-dev2
++ curl --retry 3 --location https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/tags/v1.30.0-dev2
...
++ jq -r '.assets[] | select(.name | contains("filecoin-ffi-Linux-x86_64-standard")) | .url'
+ local __release_url=https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/assets/193160603
++ basename https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/assets/193160603
+ local __tar_path=/tmp/filecoin-ffi-Linux-x86_64-standard_193160603.tar.gz
+ [[ -z https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/assets/193160603 ]]
++ curl --head --retry 3 --header Accept:application/octet-stream --location --output /dev/null -w '%{url_effective}' https://api.github.com/repos/filecoin-project/filecoin-ffi/releases/assets/193160603
...

leading to a successful install

[galargh]

Post-merge LGTM 🥇