Use gh auth token for default GITHUB_TOKEN secret

[ChristopherHX]

• initial version

Closes nektos/gh-act#8

The readEnvs function should be rewritten to apply ToUpper for the secrets keys (not for env variables) like for newSecrets so the following can get refactored to just check for GITHUB_TOKEN

		hasGitHubToken := false
		for k := range secrets {
			if strings.EqualFold(k, "GITHUB_TOKEN") {
				hasGitHubToken = true
			}
		}

I considered to not make this conditionally and skipped the ghes auto detection. Now act will always have a ${{ github.token }} value if gh is installed and authenticated.

[ChristopherHX]

act -s GITHUB_TOKEN= should disable this feature (like putting in the secrets file) if I'm not mistaken

[codecov]

Codecov Report

Attention: Patch coverage is 51.51515% with 16 lines in your changes missing coverage. Please review.

Project coverage is 68.58%. Comparing base (5a80a04) to head (1849cad).
Report is 163 commits behind head on master.

Files with missing lines	Patch %	Lines
cmd/root.go	0.00%	8 Missing ⚠️
pkg/gh/gh.go	68.00%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2651      +/-   ##
==========================================
+ Coverage   61.56%   68.58%   +7.02%     
==========================================
  Files          53       72      +19     
  Lines        9002    11046    +2044     
==========================================
+ Hits         5542     7576    +2034     
+ Misses       3020     2900     -120     
- Partials      440      570     +130     

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