Exit earlier when not overwriting output

[woot000]

Stop the program from running earlier in parse_cli() when not overwriting the output file, so that no Vapoursynth cache file is created in the case that no video encoding occurs

Incidentally, this also speeds up Av1an exiting when -n is specified, even when the Vapoursynth cache file was already created (on Windows 11: ~90ms w/ this change, ~140ms on previous commit when the Vapoursynth cache file exists, even longer if the cache file doesn't exist)

Fixes #1084

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.26%. Comparing base (f6c37bd) to head (fbcbba5).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
av1an/src/main.rs	25.00%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1220      +/-   ##
==========================================
+ Coverage   62.24%   62.26%   +0.01%     
==========================================
  Files          23       23              
  Lines        7332     7333       +1     
==========================================
+ Hits         4564     4566       +2     
+ Misses       2768     2767       -1     

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[shssoichiro]

Seems good. thanks!