Forward CCACHE_DIR into the build container

[bdraco]

What does this implement/fix?

Forwards CCACHE_DIR into the build container, next to the existing HOME forward. Docker only passes the variable when it is set on the runner, so workflows that do not set it see no change at all. When a workflow does set it to a path under HOME (already bind mounted), ESPHome inside the container picks it up: 2026.7.0 enables ccache for native ESP-IDF esp32 builds whenever the binary is on PATH, and the image base already ships ccache; esp8266 follows via esphome/esphome#17722. The caller pairs this with an actions/cache entry on that dir to persist objects across runs.

Proven out on ratgdo/esphome-ratgdo#658 (with the identical change to their fork of this action, ratgdo#16): esp32 build steps halved on the warm run, 296s to 148s and 281s to 142s, while the esp8266 boards were unaffected as expected.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (change that breaks existing workflows using this action)
• Dependency or CI update
• Documentation update

Related issue (if applicable): fixes

Checklist

• I have run the unit tests locally (python -m unittest discover -s tests) and they pass.
• Tests in tests/ have been added or updated to cover the change (if applicable).
• README.md has been updated for any new or changed inputs/outputs (if applicable).

[bdraco]

thanks