Fix: Graph deleted before aptly exits

[reglim]

Fixes: #1213

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

The temporary output file is now only deleted after copying it to the output location.
We no longer wait 1s for the open command, but exit immediately, since the process will continue running even after aptly exits.

Why this change is important?

Because the assumption was made that any application that opened the image would continue displaying it, even if the image was removed right after, which isn't true for the Gnome Image Viewer.

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.

Project coverage is 74.86%. Comparing base (b7e91f0) to head (4be2935).

Files with missing lines	Patch %	Lines
cmd/graph.go	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1214      +/-   ##
==========================================
+ Coverage   74.82%   74.86%   +0.03%     
==========================================
  Files         157      157              
  Lines       18176    18168       -8     
==========================================
  Hits        13601    13601              
+ Misses       3468     3460       -8     
  Partials     1107     1107              

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