github: Enable retention periods for uploaded artifacts.

This prevents Zulip CI from eventually consuming large amounts of
storage on one's GitHub account.

I picked a longer retention period for the Puppeteer artifacts because
humans look at those; the production tarballs are unlikely to be used
10 minutes after the run completes as they are just for the next stage
fo the build; certainly 14 days seems ample for any debugging.

.github/workflows/production-suite.yml

@@ -97,6 +97,7 @@ jobs:
         with:
           name: production-tarball
           path: /tmp/production-build
+          retention-days: 14
 
   production_install:
     strategy:

.github/workflows/zulip-ci.yml

@@ -148,6 +148,7 @@ jobs:
         with:
           name: puppeteer
           path: ./var/puppeteer
+          retention-days: 60
 
       - name: Check development database build
         if: ${{ matrix.is_focal }}