Apply clang-format universally.

* It is tough to contribute to a project that doesn't use a formatter,
* It is extra hard to contribute to a project which has configured the formatter, but ignores it for some files
* Code formatting makes it harder to hide obscure / weird bugs by accident or on purpose,

Let's rip the bandaid off?

Note that PRs currently in flight should be able to be merged relatively easily by applying `clang-format` to their tip prior to merge.

src/libfetchers-tests/access-tokens.cc

@@ -19,6 +19,7 @@ public:
     {
         experimentalFeatureSettings.experimentalFeatures.get().insert(Xp::Flakes);
     }
+
     void TearDown() override {}
 };
 
@@ -98,4 +99,4 @@ TEST_F(AccessKeysTest, multipleSourceHut)
     ASSERT_EQ(token, "token");
 }
 
-}
+} // namespace nix::fetchers