ci: cache python install to better deal with download errors. (#34408)

* ci: cache python install to better deal with download errors.

* chore: use our CDN to download python2

* build: DRY up the python install steps

* fixup backport for 19-x-y

* trigger ci

* fixup the fixup

Co-authored-by: John Kleinschmidt <jkleinsc@electronjs.org>
Co-authored-by: Samuel Attard <sattard@salesforce.com>
Co-authored-by: Cheng Zhao <zcbenz@gmail.com>

.circleci/config/base.yml

@@ -527,15 +527,6 @@ step-install-gnutar-on-mac: &step-install-gnutar-on-mac
         ln -fs /usr/local/bin/gtar /usr/local/bin/tar
       fi
 
-step-install-python2-on-mac: &step-install-python2-on-mac
-  run:
-    name: Install python2 on macos
-    command: |
-      if [ "`uname`" == "Darwin" ]; then
-        curl -O https://www.python.org/ftp/python/2.7.18/python-2.7.18-macosx10.9.pkg
-        sudo installer -pkg python-2.7.18-macosx10.9.pkg -target /
-      fi
-
 step-gn-gen-default: &step-gn-gen-default
   run:
     name: Default GN gen
@@ -1024,7 +1015,7 @@ steps-electron-gn-check: &steps-electron-gn-check
     - *step-checkout-electron
     - *step-depot-tools-get
     - *step-depot-tools-add-to-path
-    - *step-install-python2-on-mac
+    - install-python2-mac
     - *step-setup-env-for-build
     - *step-setup-goma-for-build
     - *step-generate-deps-hash
@@ -1048,7 +1039,7 @@ steps-native-tests: &steps-native-tests
     - attach_workspace:
         at: .
     - *step-depot-tools-add-to-path
-    - *step-install-python2-on-mac
+    - install-python2-mac
     - *step-setup-env-for-build
     - *step-setup-goma-for-build
     - *step-wait-for-goma
@@ -1190,6 +1181,31 @@ steps-test-node: &steps-test-node
 
 # Command Aliases
 commands:
+  install-python2-mac:
+    steps:
+      - restore_cache:
+          keys:
+            - v2.7.18-python-cache-{{ arch }}
+          name: Restore python cache
+      - run:
+          name: Install python2 on macos
+          command: |
+            if [ "`uname`" == "Darwin" ]; then
+              if [ ! -f "python-downloads/python-2.7.18-macosx10.9.pkg" ]; then
+                mkdir python-downloads
+                echo 'Downloading Python 2.7.18'
+                curl -O https://dev-cdn.electronjs.org/python/python-2.7.18-macosx10.9.pkg
+                mv python-2.7.18-macosx10.9.pkg python-downloads
+              else
+                echo 'Using Python install from cache'
+              fi
+              sudo installer -pkg python-downloads/python-2.7.18-macosx10.9.pkg -target /        
+            fi
+      - save_cache:
+          paths:
+            - python-downloads
+          key: v2.7.18-python-cache-{{ arch }}
+          name: Persisting python cache
   maybe-restore-portaled-src-cache:
     parameters:
       halt-if-successful:
@@ -1356,7 +1372,7 @@ commands:
             - run: rm -rf src/electron
       - *step-restore-brew-cache
       - *step-install-gnutar-on-mac
-      - *step-install-python2-on-mac
+      - install-python2-mac
       - *step-save-brew-cache
       - when:
           condition: << parameters.build >>
@@ -1548,7 +1564,7 @@ commands:
             - *step-depot-tools-get
       - *step-depot-tools-add-to-path
       - *step-restore-brew-cache
-      - *step-install-python2-on-mac
+      - install-python2-mac
       - *step-get-more-space-on-mac
       - when:
           condition: << parameters.checkout >>