release-lib: Don't use tryEval for packagePlatforms

This use of tryEval causes hydra to fully ignore evaluation failures of
packages that occur while trying to evaluate the hydra platforms it should be
built on. This includes failures that occur during evaluation of:
- The `.type` attribute value
- The `.meta.hydraPlatforms` or `.meta.platforms` attribute value
  - The `.version` attribute, since this can determine whether
    `.meta.position` is set
- For non-derivations, `.recurseForDerivations` or `.recurseForRelease`

Here's a minimal `release.nix` file, showcasing how a `.version` failure
is ignored:

  let
    packages = pkgs: {
      success = pkgs.stdenv.mkDerivation {
        name = "success";
      };
      ignoredFailure = pkgs.stdenv.mkDerivation {
        pname = "ignored-failure";
        version = throw "version error";
      };
      caughtFailure = pkgs.stdenv.mkDerivation {
        name = "caught-failure";
        src = throw "src error";
      };
    };

    releaseLib = import <nixpkgs/pkgs/top-level/release-lib.nix> {
      packageSet = args: packages (import <nixpkgs> args);
      supportedSystems = [ "x86_64-linux" ];
    };
  in
  releaseLib.mapTestOn (releaseLib.packagePlatforms releaseLib.pkgs)

Evaluating this with `hydra-eval-jobs` before this change yields:

  $ hydra-eval-jobs release.nix -I nixpkgs=/path/to/nixpkgs
  warning: `--gc-roots-dir' not specified
  error: "error: --- ThrownError --- hydra-eval-jobs\nsrc error"
  {
    "caughtFailure.x86_64-linux": {
      "error": "error: --- ThrownError --- hydra-eval-jobs\nsrc error"
    },
    "success.x86_64-linux": {
      "description": "",
      "drvPath": "/nix/store/q1sw933xd9bxfx6rcp0kqksbprj1wmwj-success.drv",
      "homepage": "",
      "isChannel": false,
      "license": "",
      "maintainers": "",
      "maxSilent": 7200,
      "nixName": "success",
      "outputs": {
        "out": "/nix/store/7awrz6hss4jjxvgbwi4wlyikncmslb7a-success"
      },
      "schedulingPriority": 100,
      "system": "x86_64-linux",
      "timeout": 36000
    }
  }

Where you can see that there is no job for the `ignoredFailure`
derivation. Compare this to after this change:

  $ hydra-eval-jobs release.nix -I nixpkgs=/path/to/nixpkgs
  warning: `--gc-roots-dir' not specified
  error: "error: --- ThrownError --- hydra-eval-jobs\nsrc error"
  error: "error: --- ThrownError --- hydra-eval-jobs\nversion error"
  {
    "caughtFailure.x86_64-linux": {
      "error": "error: --- ThrownError --- hydra-eval-jobs\nsrc error"
    },
    "ignoredFailure": {
      "error": "error: --- ThrownError --- hydra-eval-jobs\nversion error"
    },
    "success.x86_64-linux": {
      "description": "",
      "drvPath": "/nix/store/q1sw933xd9bxfx6rcp0kqksbprj1wmwj-success.drv",
      "homepage": "",
      "isChannel": false,
      "license": "",
      "maintainers": "",
      "maxSilent": 7200,
      "nixName": "success",
      "outputs": {
        "out": "/nix/store/7awrz6hss4jjxvgbwi4wlyikncmslb7a-success"
      },
      "schedulingPriority": 100,
      "system": "x86_64-linux",
      "timeout": 36000
    }
  }

Notice how `ignoredFailure` is now part of the result.

pkgs/top-level/release-lib.nix

@@ -142,15 +142,13 @@ rec {
   /* Recursively map a (nested) set of derivations to an isomorphic
      set of meta.platforms values. */
   packagePlatforms = mapAttrs (name: value:
-    let res = builtins.tryEval (
       if isDerivation value then
         value.meta.hydraPlatforms
           or (value.meta.platforms or [ "x86_64-linux" ])
       else if value.recurseForDerivations or false || value.recurseForRelease or false then
         packagePlatforms value
       else
-        []);
-    in if res.success then res.value else []
+        []
     );