ci(pr-title): Escape variables used in bash scripts

[aborgna-q]

Don't insert github expressions directly in the bash script.

• It breaks if there are [backticks] or [quotes] inside the variable.
• It's a security issue, it allows arbitrary code injections from the PR description.

The solution is to put them into env variables first.

[aborgna-q]

Failing job due to the ` in my description :P
https://github.com/CQCL/tket2/actions/runs/8923278983/job/24507186630?pr=328

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.69%. Comparing base (5d3b0c9) to head (443b852).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #328   +/-   ##
=======================================
  Coverage   84.69%   84.69%           
=======================================
  Files          35       35           
  Lines        4442     4442           
  Branches     4290     4290           
=======================================
  Hits         3762     3762           
  Misses        501      501           
  Partials      179      179           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.