ci: add `4-` prefix to cache keys for v2.4.0 upgrade

A hack to invalidate GitHub actions/cache in CI builds/tests for the v2.4.0 upgrade.

PR: #467

.github/workflows/build.yml

@@ -76,11 +76,11 @@ jobs:
             tools/
             zephyr/
             bootloader/
-          key: 3-${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('app/west.yml') }}
+          key: 4-${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('app/west.yml') }}
           restore-keys: |
-            3-${{ runner.os }}-build-${{ env.cache-name }}-
-            3-${{ runner.os }}-build-
-            3-${{ runner.os }}-
+            4-${{ runner.os }}-build-${{ env.cache-name }}-
+            4-${{ runner.os }}-build-
+            4-${{ runner.os }}-
         timeout-minutes: 2
         continue-on-error: true
       - name: West init

.github/workflows/test.yml

@@ -32,11 +32,11 @@ jobs:
             tools/
             zephyr/
             bootloader/
-          key: 3-${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('app/west.yml') }}
+          key: 4-${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('app/west.yml') }}
           restore-keys: |
-            3-${{ runner.os }}-build-${{ env.cache-name }}-
-            3-${{ runner.os }}-build-
-            3-${{ runner.os }}-
+            4-${{ runner.os }}-build-${{ env.cache-name }}-
+            4-${{ runner.os }}-build-
+            4-${{ runner.os }}-
         timeout-minutes: 2
         continue-on-error: true
       - name: West init