Update deployment workflow for fly.io

[CasperWA]

Fixes #2141

Using the v1 tag for the fly.io GitHub Action - also, renaming the workflow name (removing 'master' -> 'main').

Note, I've already updated the environment rule for fly.io on which branches may deploy to fit with the new default branch naming. (See here.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.83%. Comparing base (685adbb) to head (019b919).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2142   +/-   ##
=======================================
  Coverage   90.83%   90.83%           
=======================================
  Files          75       75           
  Lines        4824     4824           
=======================================
  Hits         4382     4382           
  Misses        442      442           

Flag	Coverage Δ
project	90.83% <ø> (ø)
validator	90.83% <ø> (ø)

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[ml-evs]

Perfect, thanks!