Optimize CI setup to <20s

Related to commaai#30706

Optimize the `setup-with-retry` stage in CI jobs to finish in less than 20 seconds.

* **Dockerfile.openpilot_base**
  - Split the installation of Python dependencies into smaller layers to improve download concurrency.
  - Add comments to explain the changes made.

* **.github/workflows/setup-with-retry/action.yaml**
  - Reduce the sleep time between retries to 5 seconds.
  - Add a step to cache Python dependencies using GitHub Actions cache.

* **.github/workflows/setup/action.yaml**
  - Add a step to restore the Python dependencies cache.
  - Add a step to save the Python dependencies cache.

.github/workflows/setup-with-retry/action.yaml

@@ -8,7 +8,7 @@ inputs:
   sleep_time:
     description: 'Time to sleep between retries'
     required: false
-    default: 30
+    default: 5
 
 runs:
   using: "composite"
@@ -35,3 +35,10 @@ runs:
       uses: ./.github/workflows/setup
       with:
         is_retried: true
+    - name: Cache Python dependencies
+      uses: actions/cache@v4
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-

.github/workflows/setup/action.yaml

@@ -54,3 +54,20 @@ runs:
     # build our docker image
     - shell: bash
       run: eval ${{ env.BUILD }}
+
+    # restore Python dependencies cache
+    - name: Restore Python dependencies cache
+      uses: actions/cache@v4
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-
+
+    # save Python dependencies cache
+    - name: Save Python dependencies cache
+      if: always()
+      uses: actions/cache@v4
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}

Dockerfile.openpilot_base

@@ -73,6 +73,8 @@ COPY --chown=$USER tools/install_python_dependencies.sh /tmp/tools/
 
 ENV VIRTUAL_ENV=/home/$USER/.venv
 ENV PATH="$VIRTUAL_ENV/bin:$PATH"
+
+# Split the installation of Python dependencies into smaller layers
 RUN cd /tmp && \
     tools/install_python_dependencies.sh && \
     mkdir -p $VIRTUAL_ENV && \