nixos/test-driver: mention the elapsed time when it times out

For now you had to know that the actions are retried for 900s when
seeing an error like

> Traceback (most recent call last):
>   File "/nix/store/dbvmxk60sv87xsxm7kwzzjm7a4fhgy6y-nixos-test-driver/bin/.nixos-test-driver-wrapped", line 927, in run_tests
>     exec(tests, globals())
>   File "<string>", line 1, in <module>
>   File "<string>", line 31, in <module>
>   File "/nix/store/dbvmxk60sv87xsxm7kwzzjm7a4fhgy6y-nixos-test-driver/bin/.nixos-test-driver-wrapped", line 565, in wait_for_file
>     retry(check_file)
>   File "/nix/store/dbvmxk60sv87xsxm7kwzzjm7a4fhgy6y-nixos-test-driver/bin/.nixos-test-driver-wrapped", line 142, in retry
>     raise Exception("action timed out")
> Exception: action timed out

in your (hydra) build failure. Due to the absence of timestamps you were
left guessing if the machine was just slow, someone passed a low timeout
value (which they couldn't until now) or whatever might have happened.

By making this error a bit more descriptive (by including the elapsed
time) these hopefully become more useful.

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