Fix outdated Git on Debian stretch

by enabling Debian backports

.github/workflows/test-unit.yml

@@ -41,11 +41,21 @@ jobs:
           --health-retries 5
 
     steps:
+    - name: Enable Debian backports
+      run: |
+        grep -m1 -P '^deb http://deb.debian.org/debian [a-z]+ main' /etc/apt/sources.list |
+        awk '$3 = $3"-backports"' >> /etc/apt/sources.list
+
     - name: Update operating system software repository
       run: apt-get update
 
     - name: Install operating system dependencies
-      run: apt-get install -y git zip libzip-dev
+      run: |
+        DEBIAN_RELEASE=$(
+          grep -m1 -P '^deb http://deb.debian.org/debian [a-z]+ main' /etc/apt/sources.list |
+          awk '{print $3}'
+        )
+        apt-get -t "${DEBIAN_RELEASE}-backports" install -y git zip libzip-dev
 
     - name: PECL zip
       run: docker-php-ext-install zip