fix: reuse common login flows

[sweatybridge]

What kind of change does this PR introduce?

Bug fix #1054

What is the new behavior?

Order of token parsing should be flag > env > stdin.

Additional context

Add any other context or screenshots.

[coveralls]

Pull Request Test Coverage Report for Build 8448434690

Details

• 24 of 58 (41.38%) changed or added relevant lines in 4 files are covered.
• 15 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.1%) to 57.474%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/login.go	4	5	80.0%
internal/bootstrap/bootstrap.go	0	4	0.0%
internal/login/login.go	8	37	21.62%

Files with Coverage Reduction	New Missed Lines	%
cmd/login.go	3	68.42%
internal/gen/keys/keys.go	5	11.76%
internal/login/login.go	7	27.38%

Totals
Change from base Build 8447285636:	-0.1%
Covered Lines:	6290
Relevant Lines:	10944

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💛 - Coveralls