ci: pin Windows folders, step 1 (#12597)

The mapping from Azure job to local folder is managed by a handful of
JSON files under `$WORKDIR/SourceRootMapping`. Based on the sketchy
[documentation] that screams "don't mess with these definitely internal,
subject-to-change files", it seeems to me that our best bet for pinning
Windows work folders is to mess with these files. Specifically, take a
"version" of these files we like from a live CI machine and add those
specific files to the init script of our Windows nodes.

[documentation]: https://github.com/microsoft/azure-pipelines-agent/blob/master/docs/jobdirectories.md

So the first step is to collect these files, which is what this PR aims
to accomplish.

CHANGELOG_BEGIN
CHANGELOG_END

ci/build.yml

@@ -182,6 +182,16 @@ jobs:
   steps:
     - template: report-start.yml
     - checkout: self
+    - bash: |
+        set -euo pipefail
+
+        for f in $(find /d/a/SourceRootMapping -type f); do
+          echo "-----"
+          echo $f
+          echo "-----"
+          cat $f
+          echo "-----"
+        done
     - bash: |
         set -euo pipefail
         git checkout $(release_sha)