test: cache providers for external-agent and ai-task-app-id (#25378)

Removes `SkipCacheProviders: true` from the `external-agent` and
`ai-task-app-id` subtests of `TestProvision` so they go through
`testutil.CacheTFProviders` like every other coder/coder-using test.
Today both bypass the local provider mirror and hit
`registry.terraform.io` on every CI run, which flakes on transient 5xx
responses. The flag was originally added in #19286 as a workaround for
an unreleased provider build; that workaround has been stale since
`coder/coder v2.10.0` shipped.

This won't fix the flake outright. The first run after merge and any
future test-name/file-content change are still cache misses that go to
the registry. But in steady state on `main` the Actions cache will be
hit and `terraform init` runs fully offline, which should drop registry
exposure from ~every run to the small handful of cache-miss cases.

Relates to https://github.com/coder/internal/issues/1353
Relates to https://github.com/coder/internal/issues/1193

provisioner/terraform/provision_test.go

@@ -1013,7 +1013,6 @@ func TestProvision(t *testing.T) {
 				}},
 				HasExternalAgents: true,
 			},
-			SkipCacheProviders: true,
 		},
 		{
 			Name: "ai-task-app-id",
@@ -1046,7 +1045,6 @@ func TestProvision(t *testing.T) {
 				},
 				HasAiTasks: true,
 			},
-			SkipCacheProviders: true,
 		},
 		{
 			Name: "malicious-tar",