Pin platform to 6.11.0

Build issue on 6.12.0 (latest) that just started showing up:

```plaintext
fatal: not a git repository (or any of the parent directories): .git
```

This appears to be due to unbound dependency in
idf-component-manager, see this issue:

espressif/idf-component-manager#97

Unfortunately platformio trails behind esp-idf releases, so we don't
have a simple workaround that I can figure out. Just pin to older
version for now.

Author: noahp

platformio.ini

@@ -13,7 +13,7 @@
 build_cache_dir = .build_cache
 
 [env]
-platform = espressif32
+platform = espressif32@6.11.0
 board = esp32-c3-devkitc-02
 
 ; Strictly specify the versions used to build this project