Before using yamllint on newly changed files, filter by extension

This needs to be done in addition to filter within the .yamllint file because CI runs yamllint on individual files and not directories.
The configuration within the .yamllint works when yamllint is given a directory.

Author: vojtapolasek

.github/workflows/ci_lint.yml

@@ -36,7 +36,7 @@ jobs:
       - name: Run yamllint on files modified by the PR
         run: |
           exit_code=0
-          for file in $(cat filenames.txt); do
+          for file in $(grep -E '\.(yml|yaml|fmf|profile)$' filenames.txt); do
             if [[ ! -f "$file" ]]; then
               continue
             fi