fix: Set IDF_PATH explicitly in build job

The espressif/idf:v5.5 container fails to auto-detect IDF_PATH
from export.sh when run under Gitea Actions. Set it explicitly
as an env var.

.gitea/workflows/lint.yml

@@ -20,6 +20,7 @@ jobs:
     env:
       CCACHE_DIR: /ccache
       IDF_CCACHE_ENABLE: 1
+      IDF_PATH: /opt/esp/idf
     steps:
       - name: Checkout
         run: |