Fix nix log with CA derivations

Fix #6209

When trying to run `nix log <installable>`, try first to resolve the derivation pointed to
by `<installable>` as it is the resolved one that holds the build log.

This has a couple of shortcomings:
1. It’s expensive as it requires re-reading the derivation
2. It’s brittle because if the derivation doesn’t exist anymore or can’t
   be resolved (which is the case if any one of its build inputs is missing),
   then we can’t access the log anymore

However, I don’t think we can do better (at least not right now).
The alternatives I see are:
1. Copy the build log for the un-resolved derivation. But that means a
   lot of duplication
2. Store the results of the resolving in the db. Which might be the best
   long-term solution, but leads to a whole new class of potential
   issues.

src/libstore/binary-cache-store.cc

@@ -504,18 +504,10 @@ void BinaryCacheStore::addSignatures(const StorePath & storePath, const StringSe
 
 std::optional<std::string> BinaryCacheStore::getBuildLog(const StorePath & path)
 {
-    auto drvPath = path;
-
-    if (!path.isDerivation()) {
-        try {
-            auto info = queryPathInfo(path);
-            // FIXME: add a "Log" field to .narinfo
-            if (!info->deriver) return std::nullopt;
-            drvPath = *info->deriver;
-        } catch (InvalidPath &) {
-            return std::nullopt;
-        }
-    }
+    auto maybePath = getBuildDerivationPath(path);
+    if (!maybePath)
+        return std::nullopt;
+    auto drvPath = maybePath.value();
 
     auto logPath = "log/" + std::string(baseNameOf(printStorePath(drvPath)));