Add fallback mechanism for prek executable in git hooks

[j178]

The generated hook script now checks if the full prek path is executable
and falls back to using "prek" from PATH if not available.

Closes #786

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.36%. Comparing base (db190aa) to head (87ed35d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #850   +/-   ##
=======================================
  Coverage   89.35%   89.36%           
=======================================
  Files          59       59           
  Lines       10967    10972    +5     
=======================================
+ Hits         9800     9805    +5     
  Misses       1167     1167           

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