[ci] fix valgrind workflow (#6816)

Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

.github/workflows/optional_checks.yml

@@ -16,6 +16,7 @@ jobs:
           fetch-depth: 5
           submodules: false
       - name: Check that all tests succeeded
+        shell: bash
         run: |
             workflows=(
                 "R valgrind tests;r-valgrind"
@@ -23,7 +24,7 @@ jobs:
             for i in "${workflows[@]}"; do
                 workflow_name=${i%;*}
                 comment="The last reported status from workflow \"$workflow_name\" is failure."
-                comment+=" Commit fixes and rerun the workflow."
+                comment="${comment} Commit fixes and rerun the workflow."
                 trigger_phrase=${i#*;}
                 python \
                     "$GITHUB_WORKSPACE/.ci/get-workflow-status.py" \

.github/workflows/r_valgrind.yml

@@ -32,13 +32,14 @@ jobs:
           ref: "refs/pull/${{ github.event.client_payload.pr_number }}/merge"
       - name: Send init status
         if: ${{ always() }}
+        shell: bash
         run: |
           $GITHUB_WORKSPACE/.ci/set-commit-status.sh \
             "${{ github.workflow }}" \
             "pending" \
             "${{ github.event.client_payload.pr_sha }}"
           comment="Workflow **${{ github.workflow }}** has been triggered! 🚀\r\n"
-          comment+="${GITHUB_SERVER_URL}/microsoft/LightGBM/actions/runs/${GITHUB_RUN_ID}"
+          comment="${comment}${GITHUB_SERVER_URL}/microsoft/LightGBM/actions/runs/${GITHUB_RUN_ID}"
           $GITHUB_WORKSPACE/.ci/append-comment.sh \
             "${{ github.event.client_payload.comment_number }}" \
             "${comment}"