Update last modified time of unchanged files

Set the last modified time of any files not changed on this branch to that of the last commit on the default branch. Unchanged files should then have a last modified time from before the last build of the default branch. That means we should be able to do an incremental build for only files that changed on this branch.

.github/workflows/build.yml

@@ -18,6 +18,17 @@ jobs:
       VCPKG_BINARY_SOURCES: 'clear;nuget,GitHub,readwrite'
 
     steps:
+    - uses: actions/checkout@v4
+      with:
+        ref: main
+    - name: Set file modification times
+      shell: bash
+      run: |
+        # Use committer time from head of default branch
+        commitTime=$(git show --pretty=format:%ct)
+        # Set all file modification times to commit time
+        find "*" -type f -exec touch -d "$commitTime" {} \;
+        # When branch is checked out, any files modified on that branch will have a newer time
     - uses: actions/checkout@v4
 
     - name: Add MSBuild to PATH