fix: reorganise commands/variable/update like commands/variable/set

Description

Harmonize the logic of --group or repository mode between the glab variable set and glab variable update commands.

Allows to run the update command for a group, from outside of a git repository.

Related Issues

Resolves #7455 (closed)

How has this been tested?

I downloaded the windows build from the CI/CD and tested it locally from outside a git repository.

$ glab variable update <redacted> <redacted>
  # ⇒ globally installed glab fails

$ ./glab.exe variable update <redacted> <redacted>
  # ⇒ new glab from CI works

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap