testsuite: allow root jobs for t2812-flux-job-last.t

Problem: t2812-flux-job-last.t needs to submit a job as root in order
to test handling of uid=0 in the history plugin, but job-ingest denies
all root jobs by default.

Reload the job-ingest module with `allow-root-jobs` to bypass the
ingest error and allow the test.

Author: grondo

t/t2812-flux-job-last.t

@@ -66,6 +66,9 @@ submit_as_root()
 
 # issue #5475
 # Execution may fail but submission should work - enough for this test
+test_expect_success FLUX_SECURITY 'reload job-ingest with allow-root-jobs' '
+	flux module reload job-ingest allow-root-jobs
+'
 test_expect_success FLUX_SECURITY 'run a job as fake root' '
 	submit_as_root true &&
 	FLUX_HANDLE_USERID=0 flux job last