refactor: set default environment variables format in plugin

[appleboy]

• Add INPUT_ENVS_FORMAT to the list of environment variables in main.go
• Define envsFormat variable in plugin.go
• Add default value for Config.EnvsFormat in plugin.go Exec() function

fix https://github.com/appleboy/drone-ssh/pull/235

[codecov-commenter]

Codecov Report

Merging #253 (46f9096) into master (4aabfc9) will increase coverage by 0.24%. The diff coverage is 50.00%.

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@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   37.85%   38.10%   +0.24%     
==========================================
  Files           2        2              
  Lines         420      433      +13     
==========================================
+ Hits          159      165       +6     
- Misses        261      268       +7     

Impacted Files	Coverage Δ
main.go	0.00% <0.00%> (ø)
plugin.go	100.00% <100.00%> (ø)

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