CI: Node 24 actions, pip on PEP 668 runner images, drop committed build cache

- actions/checkout@v7 and tj-actions/changed-files@v47 (Node 20 is deprecated)
- PIP_BREAK_SYSTEM_PACKAGES for Gitea's runner image (externally-managed Python)
- untrack common/.esphome (a gitlink with no submodule mapping) and ignore
  .esphome/ in every directory

.gitignore

@@ -2,7 +2,7 @@
 # This is an example and may include too much for your use-case.
 # You can modify this file to suit your needs.
 /.claude/
-/.esphome/
+.esphome/
 /secrets.yaml
 /archive/
 /trash/