stdenv: Avoid allocating intermediate attrset when checking meta validity

This is a small performance optimization. It should be impercetible to most.

Benchmarks:

- Before
``` json
{
  "cpuTime": 0.2777960002422333,
  "envs": {
    "bytes": 3832648,
    "elements": 189513,
    "number": 144784
  },
  "gc": {
    "heapSize": 402915328,
    "totalBytes": 50229344
  },
  "list": {
    "bytes": 655304,
    "concats": 3249,
    "elements": 81913
  },
  "nrAvoided": 218962,
  "nrFunctionCalls": 127718,
  "nrLookups": 40946,
  "nrOpUpdateValuesCopied": 1563978,
  "nrOpUpdates": 8542,
  "nrPrimOpCalls": 113032,
  "nrThunks": 329605,
  "sets": {
    "bytes": 29774864,
    "elements": 1824537,
    "number": 36392
  },
  "sizes": {
    "Attr": 16,
    "Bindings": 16,
    "Env": 16,
    "Value": 24
  },
  "symbols": {
    "bytes": 235909,
    "number": 24432
  },
  "values": {
    "bytes": 9691392,
    "number": 403808
  }
}
```

- After
```
{
  "cpuTime": 0.2615779936313629,
  "envs": {
    "bytes": 3833832,
    "elements": 189661,
    "number": 144784
  },
  "gc": {
    "heapSize": 402915328,
    "totalBytes": 50212960
  },
  "list": {
    "bytes": 655304,
    "concats": 3249,
    "elements": 81913
  },
  "nrAvoided": 218814,
  "nrFunctionCalls": 127718,
  "nrLookups": 40798,
  "nrOpUpdateValuesCopied": 1563978,
  "nrOpUpdates": 8542,
  "nrPrimOpCalls": 113032,
  "nrThunks": 329457,
  "sets": {
    "bytes": 29765392,
    "elements": 1824093,
    "number": 36244
  },
  "sizes": {
    "Attr": 16,
    "Bindings": 16,
    "Env": 16,
    "Value": 24
  },
  "symbols": {
    "bytes": 235909,
    "number": 24432
  },
  "values": {
    "bytes": 9687840,
    "number": 403660
  }
}
```

pkgs/stdenv/generic/check-meta.nix

@@ -479,16 +479,21 @@ let
 
   assertValidity = { meta, attrs }: let
       validity = checkValidity attrs;
-    in validity // {
+      inherit (validity) valid;
+  in validity // {
       # Throw an error if trying to evaluate a non-valid derivation
       # or, alternatively, just output a warning message.
       handled =
-        {
-          no = handleEvalIssue { inherit meta attrs; } { inherit (validity) reason errormsg; };
-          warn = handleEvalWarning { inherit meta attrs; } { inherit (validity) reason errormsg; };
-          yes = true;
-        }.${validity.valid};
-
+        (
+          if valid == "yes" then true
+          else if valid == "no" then (
+            handleEvalIssue { inherit meta attrs; } { inherit (validity) reason errormsg; }
+          )
+          else if valid == "warn" then (
+            handleEvalWarning { inherit meta attrs; } { inherit (validity) reason errormsg; }
+          )
+          else throw "Unknown validitiy: '${valid}'"
+        );
   };
 
 in { inherit assertValidity commonMeta; }