file-cache: use the right exception type for parse error

```
error: [json.exception.parse_error.101] parse error at 195: syntax error - invalid string: ill-formed UTF-8 byte; last read: '"bokml'; expected string literal
```

This is most probably caused by the upgrade in nixos-org-configurations/1b74e019b166d37a1510a99aeea86a0fa656854a
that, in turn, caused nlohmann_json to go from 2.1.0 -> 3.2.0

I haven't found concrete evidence in the changelog as to a change
changing the exception type. Though, the timing for failures matches
with this update.

Furthermore, once this fix is applied, launching the update locally
works fine.

file-cache.hh

@@ -140,7 +140,7 @@ public:
                     throw Error("NAR index for ‘%s’ has an unsupported version", storePath);
 
                 recurse("", ls.at("root"));
-            } catch (std::invalid_argument & e) {
+            } catch (json::parse_error & e) {
                 // FIXME: some filenames have non-UTF8 characters in them,
                 // which is not supported by nlohmann::json. So we have to
                 // skip the entire package.