fix: use hardcoded signing key

[sweatybridge]

What kind of change does this PR introduce?

Follow up #4688

What is the new behavior?

Initialise a hardcoded signing key so it doesn't change between commands.

Allows bearer jwt to be generated from local signing key.

$ supabase gen bearer-jwt --role service_role
Enter your signing key in JWK format (or leave blank to use local default):

Additional context

Add any other context or screenshots.

[coveralls]

Pull Request Test Coverage Report for Build 20781553961

Details

• 2 of 4 (50.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.006%) to 56.016%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/gen/bearerjwt/bearerjwt.go	2	4	50.0%

Files with Coverage Reduction	New Missed Lines	%
internal/gen/keys/keys.go	5	12.9%

Totals
Change from base Build 20781314647:	-0.006%
Covered Lines:	6848
Relevant Lines:	12225

---

💛 - Coveralls

[mateuszbasiak]

Thank you @sweatybridge for quick action! Error is now gone and the reset command works for me as expected ✅