Switch zip to zip_equal where possible (#6389)

There are two uses I'm not converting here, that seem to want the
"shortest" behavior. For everything else, I'm going to `zip_equal` since
it's more restrictive.

I wish `zip` were named `zip_shortest`.

toolchain/driver/runtimes_cache_test.cpp

@@ -251,7 +251,7 @@ TEST_F(RuntimesCacheTest, BasicBuild) {
   }
 
   for (const auto& [target, built_runtimes_path] :
-       llvm::zip(targets, built_runtimes_paths)) {
+       llvm::zip_equal(targets, built_runtimes_paths)) {
     SCOPED_TRACE(target);
     auto lookup_result = cache_.Lookup({.target = target});
     ASSERT_THAT(lookup_result, IsSuccess(_));