check-changeset.yml: replace 'echo' with 'cat << [heredoc]' to avoid interpolation of special characters in CHANGESET_ERROR_MESSAGE

This fixes the following error in the "Print changeset checker output" step:

```
/home/runner/work/_temp/05ad4244-c741-4473-b5fa-88197e013f3c.sh: line 1: fg: no job control
/home/runner/work/_temp/05ad4244-c741-4473-b5fa-88197e013f3c.sh: command substitution: line 1: unexpected EOF while looking for matching `''
/home/runner/work/_temp/05ad4244-c741-4473-b5fa-88197e013f3c.sh: command substitution: line 2: syntax error: unexpected end of file
/home/runner/work/_temp/05ad4244-c741-4473-b5fa-88197e013f3c.sh: line 1: .%0A: command not found
- Changeset formatting error in following file:%0A    changeset addchangeset add --empty%0A
```

e.g. https://github.com/firebase/firebase-js-sdk/actions/runs/7586786834/job/20665754718

Author: dconeybe

.github/workflows/check-changeset.yml

@@ -47,7 +47,10 @@ jobs:
         yarn ts-node-script scripts/ci/check_changeset.ts
       id: check-changeset
     - name: Print changeset checker output
-      run: echo "${{steps.check-changeset.outputs.CHANGESET_ERROR_MESSAGE}}"
+      run: |
+        cat << 'eof_delimiter_that_will_never_occur_in_CHANGESET_ERROR_MESSAGE'
+        ${{steps.check-changeset.outputs.CHANGESET_ERROR_MESSAGE}}
+        eof_delimiter_that_will_never_occur_in_CHANGESET_ERROR_MESSAGE
     - name: Print blocking failure status
       run: echo "${{steps.check-changeset.outputs.BLOCKING_FAILURE}}"
     - name: Find Comment