`spack solve`: use consistent units for time

`spack solve` is supposed to show you times you can compare. setup, ground, solve, etc.
all in a list. You're also supposed to be able to compare easily across runs. With
`pretty_seconds()` (introduced in #33900), it's easy to miss the units, e.g., spot the
bottleneck here:

```console
> spack solve --timers tcl
    setup        22.125ms
    load         16.083ms
    ground        8.298ms
    solve       848.055us
    total        58.615ms
```

It's easier to see what matters if these are all in the same units, e.g.:

```
> spack solve --timers tcl
    setup         0.0147s
    load          0.0130s
    ground        0.0078s
    solve         0.0008s
    total         0.0463s
```

And the units won't fluctuate from run to run as you make changes.

-[x] make `spack solve` timings consistent like before

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