[ci] automate building CRAN package (#3497)

* [ci] automate building CRAN package

* add workflow for building CRAN package

* remove newlines

* add to R README

* Apply suggestions from code review
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

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

.github/workflows/r_artifacts.yml

+name: R artifact builds
+
+on:
+  pull_request_review_comment:
+    types: [created]
+
+jobs:
+  cran-package:
+    name: cran-package
+    if: github.event.comment.body == '/gha build r-artifacts' && contains('OWNER,MEMBER,COLLABORATOR', github.event.comment.author_association)
+    timeout-minutes: 60
+    runs-on: ubuntu-latest
+    container: rocker/r-base
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v1
+        with:
+          fetch-depth: 5
+          submodules: true
+      - name: Build package
+        shell: bash
+        id: build_package
+        run: |
+          sh build-cran-package.sh || exit -1
+          LGB_VERSION=$(cat VERSION.txt | sed "s/rc/-/g")
+          ARTIFACT_NAME=lightgbm-${LGB_VERSION}-r-cran.tar.gz
+          mv lightgbm_${LGB_VERSION}.tar.gz ${ARTIFACT_NAME}
+          echo "::set-output name=artifact_name::${ARTIFACT_NAME}"
+          echo "::set-output name=artifact_path::$(pwd)/${ARTIFACT_NAME}"
+      - name: Upload package
+        uses: actions/upload-artifact@v2
+        with:
+          name: ${{ steps.build_package.outputs.artifact_name }}
+          path: ${{ steps.build_package.outputs.artifact_path }}

R-package/README.md

@@ -256,6 +256,12 @@ sh build-cran-package.sh
 
 This will create a file `lightgbm_${VERSION}.tar.gz`, where `VERSION` is the version of `LightGBM`.
 
+Alternatively, GitHub Actions can generate this file for you. On a pull request, go to the "Files changed" tab and create a comment with this phrase:
+
+> /gha build r-artifacts
+
+Go to https://github.com/microsoft/LightGBM/actions, and find the most recent run of the "R artifact builds" workflow. If it ran successfully, you'll find a download link for the package (in `.zip` format) in that run's "Artifacts" section.
+
 ### Standard Installation from CRAN Package
 
 After building the package, install it with a command like the following:
@@ -461,7 +467,7 @@ RDvalgrind \
 | cat
 ```
 
-These tests can also be triggered on any pull request by leaving a "Comment" review with the following comment:
+These tests can also be triggered on any pull request by leaving a review on the "Files changed" tab in a pull request:
 
 > /gha run r-valgrind