CI: Enable automated builds for pull requests against our master branch.

This includes branches from forked repos.

Packages and debug symbols are not uploaded to the website, but artifacts can be accessed by any authenticated GitHub user until they expire (current 90 days).
[skip ci]

.github/workflows/ci.yml

@@ -10,6 +10,15 @@ on:
       - 'docker/**'
       - 'CONTRIBUTING.md'
       - 'README.md'
+  pull_request:
+    branches:
+      - master
+    paths-ignore:
+      - '.github/ISSUE_TEMPLATE/**'
+      - '.github/FUNDING.yml'
+      - 'docker/**'
+      - 'CONTRIBUTING.md'
+      - 'README.md'
 
 env:
   DOWNLOAD_TOOL: curl -fLOSs --retry 2 --retry-delay 60
@@ -257,6 +266,7 @@ jobs:
         shell: bash
         env:
           GH_AUTH_KEY: ${{ secrets.GH_AUTH_KEY }}
+        if: github.event_name == 'push'
         run: |
           curl -fLSs --retry 2 --retry-delay 60 \
             https://github.com/google/breakpad/blob/master/src/tools/windows/binaries/dump_syms.exe?raw=true > dump_syms.exe