libstore/static-init: don't crash if nixBinDir exists but isn't accessible

In d2696cdd¹, libstore was made to prioritize NIX_BIN_DIR/nix (over
/proc/self/exe) if it exists. But `pathExists()` throws an exception if
lstat() returns any error other than non-existence.

Because this check happens in libstore's Settings constructor for the
static-initialized global `nix::Settings`, this case would abort Lix
well before main().

[1]: d2696cdd1e

Change-Id: I62e44c8a33b28c234900df3e4790c1badefee358

lix/libstore/globals.cc

@@ -116,7 +116,7 @@ Settings::Settings()
        you'll want to improve `getSelfExe()` to work on your platform.
      */
     std::string nixExePath = nixBinDir + "/nix";
-    if (!pathExists(nixExePath)) {
+    if (!pathAccessible(nixExePath)) {
         nixExePath = getSelfExe().value_or("nix");
     }
     buildHook.setDefault(Strings {