chore: reduce codecov volume

[toddbaert]

Wow... this has been frustrating...

There's a confluence of issues here:

• We're uploading codecov in a matrix (this is both unnecessary and can cause issues with too-many-requests), plus it makes the results confusing. I've fixed this by only running it on the first matrix index.
• The latest hash of the action uses their CLI - which requires a token
• Renovate doesn't like to (can't?) use hashes for github actions. This might be because version numbers of actions are mutable (the associated commit hash can be changed) so this isn't very helpful.

I've resolved all of the above, but it's quite an annoying situation.

[codecov]

Codecov Report

Merging #67 (f3cee2e) into main (c3c1222) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #67   +/-   ##
=======================================
  Coverage   98.90%   98.90%           
=======================================
  Files           6        6           
  Lines          91       91           
=======================================
  Hits           90       90           
  Misses          1        1           

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