Try Yarik's suggestion

Author: jwodder

.github/workflows/build-macos.yaml

@@ -405,8 +405,8 @@ jobs:
       - name: Set up environment
         run: |
           brew install exempi  # Needed by python-xmp-toolkit
-          mkdir -p /private/tmp/DLTMP
-          echo TMPDIR=/private/tmp/DLTMP >> "$GITHUB_ENV"
+          mkdir -p ~/DLTMP
+          echo TMPDIR='~/DLTMP' >> "$GITHUB_ENV"
           git config --global user.email "[email protected]"
           git config --global user.name "GitHub Almighty"
 

.github/workflows/template/build-{{ostype}}.yaml.j2

@@ -681,8 +681,8 @@ jobs:
         run: |
           {% if ostype == "macos" %}
           brew install exempi  # Needed by python-xmp-toolkit
-          mkdir -p /private/tmp/DLTMP
-          echo TMPDIR=/private/tmp/DLTMP >> "$GITHUB_ENV"
+          mkdir -p ~/DLTMP
+          echo TMPDIR='~/DLTMP' >> "$GITHUB_ENV"
           {% endif %}
           git config --global user.email "[email protected]"
           git config --global user.name "GitHub Almighty"