chore: migrate from tenv linter to usetesting linter (#20401)

The tenv linter is deprecated in favor of usetesting which offers a
superset of lint checks. This message is seen when running `make lint`

```
[nix-shell:~/src/coder]$ make lint
<snip>
WARN The linter 'tenv' is deprecated (since v1.64.0) due to: Duplicate feature in another linter. Replaced by usetesting.
<snip>
```


This change swaps out the deprecated tenv linter for the usetesting linter,
and configures it for linting parity.


https://github.com/coder/coder/issues/20398

coderd/util/tz/tz_test.go

@@ -35,12 +35,9 @@ func Test_TimezoneIANA(t *testing.T) {
 			// This test can be flaky on some Windows runners :(
 			t.Skip("This test is flaky under Windows.")
 		}
-		oldEnv, found := os.LookupEnv("TZ")
+		_, found := os.LookupEnv("TZ")
 		if found {
 			require.NoError(t, os.Unsetenv("TZ"))
-			t.Cleanup(func() {
-				_ = os.Setenv("TZ", oldEnv)
-			})
 		}
 
 		zone, err := tz.TimezoneIANA()