ability to disable cache. off by default except for...

...zig run, zig build, compiler_rt.a, and builtin.a

src/cache_hash.cpp

@@ -467,3 +467,11 @@ void cache_release(CacheHash *ch) {
     assert(ch->manifest_file_path != nullptr);
     os_file_close(ch->manifest_file);
 }
+
+Buf *get_random_basename() {
+    Buf *result = buf_alloc();
+    for (size_t i = 0; i < 16; i += 1) {
+        buf_append_char(result, base64_fs_alphabet[rand() % 64]);
+    }
+    return result;
+}