Set up the SSH agent for CI

[Michael-F-Bryan]

We accidentally broke master when #51 was merged because the fft proc block tries to pull in sonograph (a private hotg-ai repo) as a git dependency.

CI Failure Message

``` /usr/share/rust/.cargo/bin/cargo check --workspace --verbose Updating crates.io index Updating git repository `ssh://git@github.com/hotg-ai/sonogram` error: failed to get `sonogram` as a dependency of package `FFT v0.1.0 (/home/runner/work/rune/rune/proc_blocks/fft)` Error: failed to get `sonogram` as a dependency of package `FFT v0.1.0 (/home/runner/work/rune/rune/proc_blocks/fft)` Caused by: failed to load source for dependency `sonogram` Caused by: Unable to update ssh://git@github.com/hotg-ai/sonogram#39d4c460 Caused by: failed to clone into: /home/runner/.cargo/git/db/sonogram-b77ca210ffbc4b64 Caused by: failed to authenticate when downloading repository * attempted ssh-agent authentication, but no usernames succeeded: `git` if the git CLI succeeds then `net.git-fetch-with-cli` may help here https://doc.rust-lang.org/cargo/reference/config.html#netgit-fetch-with-cli Caused by: error authenticating: no auth sock variable; class=Ssh (23) ```

To let CI use other private repos we need to:

• Create a SSH key and add to the repo as a secret
• Add the webfactory/ssh-agent action to benchmarks.yml and main.yml which uses that SSH key

After that, SSH authentication should work and CI will pass again.

If it doesn't, we might need to tell cargo to shell out to git instead of using the compiled-in libssh2 by setting git-fetch-with-cli = true under the [net] table in .cargo/config.

We may also want to set up GitHub's protected branches so nobody can push directly to master and CI needs to pass before a PR can be merged.

[Michael-F-Bryan]

No longer necessary.