print type and value in "flake attr is not a derivation" errors

This turns errors like:

error: flake output attribute 'hydraJobs' is not a derivation or path

into errors like:

error: expected flake output attribute 'hydraJobs' to be a derivation or
path but found a set: { binaryTarball = «thunk»; build = «thunk»; etc> }

This change affects all InstallableFlake commands.

Source: https://git.lix.systems/lix-project/lix/commit/20981461d4a2a62c68f4bc7c4258473f7cd7d8e1
Signed-off-by: Jörg Thalheim <[email protected]>

doc/manual/rl-next/print-value-in-installable-flake-error.md

@@ -0,0 +1,18 @@
+---
+synopsis: `nix flake` commands that expect derivations now print the failing value and its type
+prs: 10778
+---
+
+In errors like `flake output attribute 'nixosConfigurations.yuki.config' is not a derivation or path`, the message now includes the failing value and type.
+
+Before:
+
+```
+    error: flake output attribute 'nixosConfigurations.yuki.config' is not a derivation or path
+````
+
+After:
+
+```
+    error: expected flake output attribute 'nixosConfigurations.yuki.config' to be a derivation or path but found a set: { appstream = «thunk»; assertions = «thunk»; boot = { bcache = «thunk»; binfmt = «thunk»; binfmtMiscRegistrations = «thunk»; blacklistedKernelModules = «thunk»; bootMount = «thunk»; bootspec = «thunk»; cleanTmpDir = «thunk»; consoleLogLevel = «thunk»; «43 attributes elided» }; «48 attributes elided» }
+```

src/libcmd/installable-flake.cc

@@ -106,9 +106,14 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
             fmt("while evaluating the flake output attribute '%s'", attrPath)))
         {
             return { *derivedPathWithInfo };
+        } else {
+            throw Error(
+                "expected flake output attribute '%s' to be a derivation or path but found %s: %s",
+                attrPath,
+                showType(v),
+                ValuePrinter(*this->state, v, errorPrintOptions)
+            );
         }
-        else
-            throw Error("flake output attribute '%s' is not a derivation or path", attrPath);
     }
 
     auto drvPath = attr->forceDerivation();