github/workflow: Fix the PR Labeling for Build System

Our New PR Labeler incorrectly labels the Changed Files for Build System. Here is a Sample PR that contains changes for Arm32 CMake and Makefile: https://github.com/lupyuen6/nuttx/pull/59

```
arch/arm/CMakeLists.txt
arch/arm/Makefile
```

But our PR Labeler incorrectly labels the above as `Area: Build system, Arch: Arm`, which triggers a Complete CI Build across All Architectures (according to arch.yml). The correct label should be `Arch: Arm`, which will trigger only the Arm32 Build: https://github.com/lupyuen8/nuttx/pull/1

This PR fixes the PR Labeling. The New PR Labeler is explained here:
- https://lupyuen.org/articles/prtarget
- https://github.com/apache/nuttx/issues/18359

`.github/workflows/labeler.yml`: Changed the Regex Pattern. Now we match the Start Of Line and End Of Line.

Signed-off-by: Lup Yuen Lee <luppy@appkaki.com>

.github/workflows/labeler.yml

@@ -108,7 +108,10 @@ jobs:
                   .split('.').join('\\.')            // .  becomes \.
                   .split('*').join('[^/]*')          // *  becomes [^/]*
                   .split('[^/]*[^/]*').join('.*');   // ** becomes .*
-                archLabels.push({ label, pattern });
+                archLabels.push({
+                  label,                        // Arch: arm
+                  pattern: '^' + pattern + '$'  // Match the Line Start and Line End
+                });
 
               } else {
                 // We don't support all rules of `actions/labeler`