fix: empty reponse on provider registry json

[mavogel]

What does this do / why do we need it?

Fixes #137

How this PR fixes the problem?

By adding the flag --custom-provider-repository-url which avoid querying the terraform registry

What should your reviewer look out for in this PR?

if the flag make sense or we'd use a fallback and prepend https://github.com/

Check lists

• Test passed
• Coding style (indentation, etc)

Which issue(s) does this PR fix?

#137

[coveralls]

Pull Request Test Coverage Report for Build 5409359974

• 17 of 22 (77.27%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 65.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/app/install.go	17	19	89.47%
internal/app/app.go	0	3	0.0%

Totals
Change from base Build 5345102601:	0.6%
Covered Lines:	338
Relevant Lines:	520

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💛 - Coveralls