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

lib/spack/spack/test/util/timer.py

@@ -120,9 +120,9 @@ def test_timer_write():
 
     output = text_buffer.getvalue().splitlines()
     assert "timer" in output[0]
-    assert "1.000s" in output[0]
+    assert "1.0000s" in output[0]
     assert "total" in output[1]
-    assert "3.000s" in output[1]
+    assert "3.0000s" in output[1]
 
     deserialized = json.loads(json_buffer.getvalue())
     assert deserialized == {

lib/spack/spack/util/timer.py

@@ -140,11 +140,11 @@ def write_json(self, out=sys.stdout):
     def write_tty(self, out=sys.stdout):
         """Write a human-readable summary of timings"""
         # Individual timers ordered by registration
-        formatted = [(p, pretty_seconds(self.duration(p))) for p in self.phases]
+        formatted = [(p, f"{self.duration(p):.4f}s") for p in self.phases]
 
         # Total time
-        formatted.append(("total", pretty_seconds(self.duration())))
+        formatted.append(("total", f"{self.duration():.4f}s"))
 
         # Write to out
         for name, duration in formatted:
-            out.write("    {:10s} {:>10s}\n".format(name, duration))
+            out.write(f"    {name:10s} {duration:>10s}\n")