disable dnsclient because it is fragile (#23728)

```
  Unhandled exception: /home/runner/work/Nim/Nim/pkgstemp/dnsclient/tests/test1.nim(28, 3) `rr.strings == @["dnsclient.nim"]`  [AssertionDefect]
  [FAILED] query TXT
  [OK] query MX
  [OK] query CNAME
  [OK] query SRV
  Error: execution of an external program failed: '/home/runner/work/Nim/Nim/pkgstemp/dnsclient/tests/test1'
         Tip: 2 messages have been suppressed, use --verbose to show them.
  tools.nim(36)            doCmd
  
      Error:  Execution failed with exit code 1
          ... Command: /home/runner/work/Nim/Nim/bin/nim c --noNimblePath -d:NimblePkgVersion=0.3.4 --hints:off -r --path:. /home/runner/work/Nim/Nim/pkgstemp/dnsclient/tests/test1 
```

testament/important_packages.nim

@@ -63,7 +63,7 @@ pkg "criterion", allowFailure = true # needs testing binary
 pkg "datamancer"
 pkg "dashing", "nim c tests/functional.nim"
 pkg "delaunay"
-pkg "dnsclient"
+pkg "dnsclient", allowFailure = true # super fragile
 pkg "docopt"
 pkg "dotenv"
 # when defined(linux): pkg "drchaos"