Fix missing newline before EOF delimiter in gh-cache/cache output

The 'Output cached value' step cats a JSON file into GITHUB_OUTPUT
followed by an EOF delimiter, but if the file has no trailing newline
the delimiter lands on the same line as the content, causing:
'Invalid value. Matching delimiter not found EOF'

Add a bare echo to ensure a newline before the closing delimiter.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lindseywild

.github/actions/gh-cache/cache/action.yml

@@ -63,6 +63,7 @@ runs:
         echo "value<<EOF" >> $GITHUB_OUTPUT
         if [ -f "${{ inputs.key }}" ]; then
           cat "${{ inputs.key }}" >> $GITHUB_OUTPUT
+          echo >> $GITHUB_OUTPUT
           echo "Outputted 'value=$(cat "${{ inputs.key }}")'"
         else
           echo "Skipped outputting 'value'"