Only match GCC warnings from ESPHome source files in CI (#2756)

.github/workflows/matchers/gcc.json

@@ -5,7 +5,7 @@
       "severity": "error",
       "pattern": [
         {
-          "regexp": "^(.*):(\\d+):(\\d+):\\s+(?:fatal\\s+)?(warning|error):\\s+(.*)$",
+          "regexp": "^src/(.*):(\\d+):(\\d+):\\s+(?:fatal\\s+)?(warning|error):\\s+(.*)$",
           "file": 1,
           "line": 2,
           "column": 3,