fix(persist-ccache-cache): rename cache_size option to ccache_maxsize

The option 'cache_size' was not being correctly mapped to the CCACHE_MAXSIZE
environment variable used by the install script. DevContainer Features
convert option names to environment variables using the pattern:
${FEATURE_ID}_${OPTION_NAME_UPPERCASE}

Changed the option from 'cache_size' to 'ccache_maxsize' to ensure correct
mapping to CCACHE_MAXSIZE in the initialization script.

Fixes the issue where ccache_maxsize setting was being ignored.

Author: ckagerer

src/persist-ccache-cache/devcontainer-feature.json

@@ -5,7 +5,7 @@
   "description": "Persist ccache compiler cache across container rebuilds.",
   "documentationURL": "https://github.com/ckagerer/devcontainer-features/tree/main/src/persist-ccache-cache",
   "options": {
-    "cache_size": {
+    "ccache_maxsize": {
       "type": "string",
       "default": "5G",
       "description": "Maximum ccache size (e.g., 5G, 10GB, 50G). Default: 5G"