ci: show main branch link text in release PR description

[mwbrooks]

Changelog

N/A - CI only.

Summary

The release PR description renders the first Testing checkbox with an empty link where main should be (see #681). The body is built with an unquoted heredoc, so the backticks around main are treated as shell command substitution and replaced with empty output.

This removes the backticks so the link text renders as plain main.

Testing

• Next run of the release workflow should render: Confirm unit tests and E2E tests are passing on the main branch.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[zimeg]

@mwbrooks Amazing catch so let's merge this? Now!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.23%. Comparing base (e70c8bc) to head (cafe3fb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #683   +/-   ##
=======================================
  Coverage   78.23%   78.23%           
=======================================
  Files         239      239           
  Lines       18149    18149           
=======================================
+ Hits        14198    14199    +1     
+ Misses       3951     3950    -1     

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