Use revcount/last-modified for computing the flake fingerprint

The store path is not enough. For example, when we build a dirty tree,
commit, and build the clean tree, a re-evaluation is necessary because
the flake may depend on the lastModified or revCount attributes.

src/libexpr/flake/flake.cc

@@ -686,7 +686,11 @@ Fingerprint ResolvedFlake::getFingerprint() const
     // and we haven't changed it, then it's sufficient to use
     // flake.sourceInfo.storePath for the fingerprint.
     return hashString(htSHA256,
-        fmt("%s;%s", flake.sourceInfo.storePath, lockFile));
+        fmt("%s;%d;%d;%s",
+            flake.sourceInfo.storePath,
+            flake.sourceInfo.revCount.value_or(0),
+            flake.sourceInfo.lastModified.value_or(0),
+            lockFile));
 }
 
 }