ci: filter usermod matrix to only build changed usermod directories

Instead of building every usermod with a library.json on each PR, use
git diff to identify which usermods/ subdirectories were actually touched
and intersect that with the known-good library.json list.

This reduces CI time significantly for PRs that only modify one or two
usermods (previously every PR triggered ~40 usermods × 4 chipsets).

Also removes the unnecessary PlatformIO install from get_usermod_envs
(the step only uses shell/jq, not pio) and adds fetch-depth: 0 to
ensure the base branch is available for the diff.

Author: netmindz

.github/workflows/usermods.yml

@@ -20,23 +20,35 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v4
-    - uses: actions/setup-python@v5
       with:
-        python-version: '3.12'
-        cache: 'pip'
-    - name: Install PlatformIO
-      run: pip install -r requirements.txt
-    - name: Get default environments
+        fetch-depth: 0
+    - name: Get changed usermod environments
       id: envs
       run: |
-        echo "usermods=$(find usermods/ -name library.json | xargs dirname | xargs -n 1 basename | jq -R | grep -v PWM_fan | grep -v BME68X_v2| grep -v pixels_dice_tray | jq --slurp -c)" >> $GITHUB_OUTPUT
+        # Usermods whose directories changed in this PR
+        changed=$(git diff --name-only origin/${{ github.base_ref }}...HEAD \
+          | grep '^usermods/' | cut -d/ -f2 | sort -u || true)
+
+        # All usermods with a library.json (excluding known-incompatible ones)
+        all=$(find usermods/ -name library.json \
+          | xargs dirname | xargs -n 1 basename \
+          | grep -v PWM_fan | grep -v BME68X_v2 | grep -v pixels_dice_tray \
+          | sort || true)
+
+        if [ -z "$changed" ] || [ -z "$all" ]; then
+          echo "usermods=[]" >> $GITHUB_OUTPUT
+        else
+          usermods=$(comm -12 <(echo "$all") <(echo "$changed") | jq -R | jq --slurp -c)
+          echo "usermods=$usermods" >> $GITHUB_OUTPUT
+        fi
     outputs:
       usermods: ${{ steps.envs.outputs.usermods }}
 
 
   build:
     # Only run for pull requests from forks (not from branches within wled/WLED)
-    if: github.event.pull_request.head.repo.full_name != github.repository
+    # Skip when no changed usermods were found (e.g. only non-library changes)
+    if: github.event.pull_request.head.repo.full_name != github.repository && needs.get_usermod_envs.outputs.usermods != '[]'
     name: Build Enviornments
     runs-on: ubuntu-latest
     needs: get_usermod_envs