flakes: check for flake.nix before complaining that lstat on it fails

getFlake currently calls lstat (via isLink via canonPath) before it
performs the sanity check that a flake.nix exists in the first place.
This commit moves the check to before path canonicalization, so that
failed symlink check operations don't throw before the check does.

src/libexpr/flake/flake.cc

@@ -212,8 +212,16 @@ static Flake getFlake(
     auto [storePath, resolvedRef, lockedRef] = fetchOrSubstituteTree(
         state, originalRef, allowLookup, flakeCache);
 
+    // We need to guard against symlink attacks, but before we start doing
+    // filesystem operations we should make sure there's a flake.nix in the
+    // first place.
+    auto unsafeFlakeDir = state.store->toRealPath(storePath) + "/" + lockedRef.subdir;
+    auto unsafeFlakeFile = unsafeFlakeDir + "/flake.nix";
+    if (!pathExists(unsafeFlakeFile))
+        throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", lockedRef, lockedRef.subdir);
+
     // Guard against symlink attacks.
-    auto flakeDir = canonPath(state.store->toRealPath(storePath) + "/" + lockedRef.subdir, true);
+    auto flakeDir = canonPath(unsafeFlakeDir, true);
     auto flakeFile = canonPath(flakeDir + "/flake.nix", true);
     if (!isInDir(flakeFile, state.store->toRealPath(storePath)))
         throw Error("'flake.nix' file of flake '%s' escapes from '%s'",
@@ -226,9 +234,6 @@ static Flake getFlake(
         .storePath = storePath,
     };
 
-    if (!pathExists(flakeFile))
-        throw Error("source tree referenced by '%s' does not contain a '%s/flake.nix' file", lockedRef, lockedRef.subdir);
-
     Value vInfo;
     state.evalFile(state.rootPath(CanonPath(flakeFile)), vInfo, true); // FIXME: symlink attack