Closes https://github.com/coder/internal/issues/609.
As seen in the below logs, the `last_used_at` time was updating, but just to the same value that it was on creation; `dbtime.Now` was called in quick succession.
```
t.go:106: 2025-05-05 12:11:54.166 [info] coderd.workspace_usage_tracker: updated workspaces last_used_at count=1 now="2025-05-05T12:11:54.161329Z"
t.go:106: 2025-05-05 12:11:54.172 [debu] coderd: GET host=localhost:50422 path=/api/v2/workspaces/745b7ff3-47f2-4e1a-9452-85ea48ba5c46 proto=HTTP/1.1 remote_addr=127.0.0.1 start="2025-05-05T12:11:54.1669073Z" workspace_name=peaceful_faraday34 requestor_id=b2cf02ae-2181-480b-bb1f-95dc6acb6497 requestor_name=testuser requestor_email="" took=5.2105ms status_code=200 latency_ms=5 params_workspace=745b7ff3-47f2-4e1a-9452-85ea48ba5c46 request_id=7fd5ea90-af7b-4104-91c5-9ca64bc2d5e6
workspaceagentsrpc_test.go:70:
Error Trace: C:/actions-runner/coder/coder/coderd/workspaceagentsrpc_test.go:70
Error: Should be true
Test: TestWorkspaceAgentReportStats
Messages: 2025-05-05 12:11:54.161329 +0000 UTC is not after 2025-05-05 12:11:54.161329 +0000 UTC
```
If we change the initial `LastUsedAt` time to be a time in the past, ticking with a `dbtime.Now` will always update it to a later value. If it never updates, the condition will still fail.
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