Quote variable contents in update checker

This job was failing in a way that suggests that because `$TZDATA_NEWS`
is a multiline string, `TZDATA_NEWS=$TZDATA_NEWS >> $GITHUB_ENV` is
creating an invalid ENV file. This is an attempt to remedy the problem.

Author: pganssle

.github/workflows/check-for-updates.yml

@@ -87,8 +87,8 @@ jobs:
           # Extract TZDATA_NEWS from file content
           TZDATA_NEWS=$(cat "$news_files")
 
-          echo "TZDATA_VERSION=$TZDATA_VERSION" >> $GITHUB_ENV
-          echo "TZDATA_NEWS=$TZDATA_NEWS" >> $GITHUB_ENV
+          echo "TZDATA_VERSION='$TZDATA_VERSION'" >> $GITHUB_ENV
+          echo "TZDATA_NEWS='$TZDATA_NEWS'" >> $GITHUB_ENV
 
       - name: Commit changes
         id: commit_changes