fix(ci): properly escape PR title

[thomaseizinger]

Description

Within double quoted strings, bash tries to evaluate everything within backticks as a command. The GitHub security guide recommends to use an intermediary environment variable instead: https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-an-intermediate-environment-variable

See https://github.com/libp2p/rust-libp2p/actions/runs/3889880383/jobs/6638520274#step:3:11.

Notes

Links to any relevant issues

Open Questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[thomaseizinger]

Draft until I have confirmed that this works.

[mxinden]

In case another example is needed: https://github.com/libp2p/rust-libp2p/actions/runs/3893266711/jobs/6645662828

[thomaseizinger]

@mxinden @jxs Friendly ping, this is ready for review (and has a series of PRs depending on it).

[thomaseizinger]

Proof that it is working: https://github.com/libp2p/rust-libp2p/actions/runs/3907325150/jobs/6676432195#step:3:10

[jxs]

LGTM