lib.formats.yaml: use well known YAML format

The way `(lib.formats.yaml {}).generate` generates YAML is compliant
because on YAML 1.2 spec JSON is a subset of YAML but it bugs people's
minds and can lead to problems with software that is not compatible with
YAML 1.2.

This commit also changes the test of the generation function. Data
validation/typing remains the same.

See #133802.

Signed-off-by: lucasew <[e-mail]>

nixos/doc/manual/from_md/release-notes/rl-2111.section.xml

···
           sign OCSP responses and server certificates.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          <literal>lib.formats.yaml</literal>’s
+          <literal>generate</literal> will not generate JSON anymore,
+          but instead use more of the YAML-specific syntax.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
 </section>

nixos/doc/manual/release-notes/rl-2111.section.md

···
 - Zfs: `latestCompatibleLinuxPackages` is now exported on the zfs package. One can use `boot.kernelPackages = config.boot.zfs.package.latestCompatibleLinuxPackages;` to always track the latest compatible kernel with a given version of zfs.
 
 - Nginx will use the value of `sslTrustedCertificate` if provided for a virtual host, even if `enableACME` is set. This is useful for providers not using the same certificate to sign OCSP responses and server certificates.
+
+- `lib.formats.yaml`'s `generate` will not generate JSON anymore, but instead use more of the YAML-specific syntax.

pkgs/pkgs-lib/formats.nix

···
 
   };
 
-  # YAML has been a strict superset of JSON since 1.2
-  yaml = {}:
-    let jsonSet = json {};
-    in jsonSet // {
-      type = jsonSet.type // {
+  yaml = {}: {
+
+    generate = name: value: pkgs.runCommand name {
+        nativeBuildInputs = [ pkgs.remarshal ];
+        value = builtins.toJSON value;
+        passAsFile = [ "value" ];
+      } ''
+        json2yaml "$valuePath" "$out"
+      '';
+
+    type = with lib.types; let
+      valueType = nullOr (oneOf [
+        bool
+        int
+        float
+        str
+        path
+        (attrsOf valueType)
+        (listOf valueType)
+      ]) // {
         description = "YAML value";
       };
-    };
+    in valueType;
+
+  };
 
   ini = {
     # Represents lists as duplicate keys

pkgs/pkgs-lib/tests/formats.nix

···
       path = ./formats.nix;
     };
     expected = ''
-      {
-        "attrs": {
-          "foo": null
-        },
-        "false": false,
-        "float": 3.141,
-        "list": [
-          null,
-          null
-        ],
-        "null": null,
-        "path": "${./formats.nix}",
-        "str": "foo",
-        "true": true
-      }
+      attrs:
+        foo: null
+      'false': false
+      float: 3.141
+      list:
+        - null
+        - null
+      'null': null
+      path: ${./formats.nix}
+      str: foo
+      'true': true
     '';
   };