break out after reading the value

This is a small optimization, it avoids reading the full file when an
early key is requested.

In the case of an *invalid* TOML file such as
```
K=80
K=81
```
this stops the value of K actually returned being 8081 and makes it 80
instead; but as it's a malformed file it doesn't really matter much.

shared-module/os/getenv.c

@@ -292,6 +292,7 @@ STATIC os_getenv_err_t os_getenv_vstr(const char *path, const char *key, vstr_t
     while (!is_eof(&active_file)) {
         if (key_matches(&active_file, key)) {
             result = read_value(&active_file, buf, quoted);
+            break;
         }
     }
     close_file(&active_file);