refactor: standardize environment variables naming conventions

[appleboy]

• Modify the EnvVars slice to include INPUT_ prefix when needed

• Remove unused environment variables from the EnvVars slice

• Update the EnvVars slice to include PROXY_ prefix when needed

• Modify the EnvVars slice to include PLUGIN_SSH_CIPHERS as PROXY_SSH_CIPHERS

• Modify the EnvVars slice to include PLUGIN_PROXY_USE_INSECURE_CIPHER as PROXY_SSH_USE_INSECURE_CIPHER

• Modify the EnvVars slice to include PLUGIN_PROXY_FINGERPRINT as PROXY_SSH_FINGERPRINT

• Remove the environment variable PROXY_HOST

• Modify the EnvVars slice to include PLUGIN_DEBUG as INPUT_DEBUG

• https://github.com/appleboy/ssh-action/issues/53#issuecomment-1184700432

• https://github.com/appleboy/ssh-action/issues/32#issuecomment-1184700363

• https://github.com/appleboy/ssh-action/issues/213

[codecov-commenter]

Codecov Report

Merging #252 (f1cf490) into master (a8d30dc) will not change coverage. The diff coverage is 0.00%.

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@@           Coverage Diff           @@
##           master     #252   +/-   ##
=======================================
  Coverage   37.85%   37.85%           
=======================================
  Files           2        2           
  Lines         420      420           
=======================================
  Hits          159      159           
  Misses        261      261           

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

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