ci: Miscellaneous improvements (#6132)

Cache downloaded tools.
Cancel duplicate jobs.
Use current repo when linking the core, this allows users to test their code when creating PRs against their forks (for instance an error in an example sketch will build successfully in a fork workflow.)
Cache Arduino IDE.
Add workflow_dispatch to be able to trigger the workflow manually.

.github/scripts/install-arduino-core-esp32.sh

@@ -15,7 +15,7 @@ if [ ! -d "$ARDUINO_ESP32_PATH" ]; then
 		pip install requests > /dev/null
 	fi
 
-	if [ "$GITHUB_REPOSITORY" == "espressif/arduino-esp32" ];  then
+	if [ ! -z "$GITHUB_REPOSITORY" ];  then
 		echo "Linking Core..."
 		ln -s $GITHUB_WORKSPACE esp32
 	else

.github/workflows/push.yml

@@ -1,12 +1,17 @@
 name: ESP32 Arduino CI
 
 on:
+  workflow_dispatch:
   push:
     branches:
     - master
     - release/*
   pull_request:
 
+concurrency:
+  group: build-${{github.event.pull_request.number || github.ref}}
+  cancel-in-progress: true
+
 jobs:
 
   # Ubuntu
@@ -22,6 +27,16 @@ jobs:
     - uses: actions/setup-python@v2
       with:
         python-version: '3.x'
+    - name: Cache tools
+      id: cache-linux
+      uses: actions/cache@v2
+      with:
+        path: |
+          ./tools/dist
+          ~/arduino_ide
+        key: ${{ runner.os }}-${{ hashFiles('package/package_esp32_index.template.json',
+                                            'tools/get.py',
+                                            '.github/scripts/install-arduino-ide.sh') }}
     - name: Build Sketches
       run: bash ./.github/scripts/on-push.sh ${{ matrix.chunk }} 15