refactor: remove unnecessary quotes in plugin build process

[appleboy]

• Modify rmcmd and mkdircmd to remove unneeded quotes
• Update buildUnTarArgs in plugin.go to use target without quotes
• Add Exec function to plugin.go with strings.Replace call
• Update tests in plugin_test.go to use target without quotes

ref: https://github.com/appleboy/scp-action/issues/112

[codecov-commenter]

Codecov Report

Merging #177 (d026aa7) into master (c663c07) will increase coverage by 0.09%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
+ Coverage   42.98%   43.07%   +0.09%     
==========================================
  Files           4        4              
  Lines         577      578       +1     
==========================================
+ Hits          248      249       +1     
  Misses        316      316              
  Partials       13       13              

Impacted Files	Coverage Δ
command.go	57.14% <100.00%> (ø)
plugin.go	84.75% <100.00%> (+0.05%)	:arrow_up:

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