fix(tvix/eval): reintroduce 'InvalidAttribuetName' error variant

As pointed out by sterni in cl/6205, this is actually possible in
syntactically valid expressions like

  { ${12 + 13} = 12; }

Change-Id: Id8a1e3aceb551f288f9050c4eea563eb6572f1a7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6461
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/errors.rs

@@ -6,6 +6,12 @@ pub enum ErrorKind {
         key: String,
     },
 
+    /// Attempted to specify an invalid key type (e.g. integer) in a
+    /// dynamic attribute name.
+    InvalidAttributeName {
+        given: &'static str,
+    },
+
     AttributeNotFound {
         name: String,
     },

tvix/eval/src/value/attrs.rs

@@ -286,7 +286,11 @@ impl NixAttrs {
                     continue;
                 }
 
-                other => panic!("unexpected attribute key: {} :: {}", other, other.type_of()),
+                other => {
+                    return Err(ErrorKind::InvalidAttributeName {
+                        given: other.type_of(),
+                    })
+                }
             }
         }