fix parsing error in --only for cosign copy

[bobcallaway]

If the --only option was omitted, the default tagSet became []string{""} which is strictly not []string{}... and parsing fails.

This switches the command line arg over to StringSliceVar and adds unit tests.

Fixes: #4048

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 36.89%. Comparing base (2ef6022) to head (25e9c4f).
Report is 297 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/cosign/cli/options/copy.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4049      +/-   ##
==========================================
- Coverage   40.10%   36.89%   -3.22%     
==========================================
  Files         155      209      +54     
  Lines       10044    13352    +3308     
==========================================
+ Hits         4028     4926     +898     
- Misses       5530     7799    +2269     
- Partials      486      627     +141     

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