fix: install just with setup action

[dev-jodee]

Summary

• Replace the brittle just.systems curl installer with extractions/setup-just@v4.
• Pin just to 1.50.0 in the shared CI setup action.

Test Plan

• git diff --check
• pre-push hook passed: just fmt-check and just lint-check

Context

• Fixes the main CI rust-integration-test setup failure where just.systems returned HTTP 403.

[github-actions]

Compute Unit Report

Instruction	Samples	Min CUs	Max CUs	Avg CUs	Est Cost (Low) [SOL]	Est Cost (Med) [SOL]	Est Cost (High) [SOL]
cancel_subscription	11	1805	2129	2013	0.000005000	0.000005080	0.000006006
close_subscription_authority	7	1881	1916	1886	0.000005000	0.000005075	0.000005943
create_fixed_delegation	38	3616	9647	5071	0.000005001	0.000005202	0.000007535
create_plan	84	3540	14053	5186	0.000005001	0.000005207	0.000007593
create_recurring_delegation	27	3640	14173	4924	0.000005001	0.000005196	0.000007462
delete_plan	8	416	416	416	0.000005000	0.000005016	0.000005208
init_subscription_authority	142	7431	22851	12332	0.000005003	0.000005493	0.000011166
revoke_delegation	19	318	585	420	0.000005000	0.000005016	0.000005210
subscribe	21	6670	12694	7751	0.000005002	0.000005310	0.000008875
transfer_fixed	6	10097	13094	10846	0.000005003	0.000005433	0.000010423
transfer_recurring	17	10206	13293	11125	0.000005003	0.000005445	0.000010562
transfer_subscription	10	10473	13473	11230	0.000005003	0.000005449	0.000010615
update_plan	21	424	503	476	0.000005000	0.000005019	0.000005238

Generated: 2026-04-29