CI: install elan in lint_and_suggest, suggest import changes (#15250)

The `lint` step in [this action](https://github.com/leanprover-community/mathlib4/actions/runs/10137312257/job/28027367787?pr=15250) used to fail.

Also added reviewdog suggestion to propose adding missing `import`s in "import all" files.

#15251 tests that the step can fail and the reviewdog suggests changes to `Mathlib.lean`.

[Zulip thread](https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/silent.20CI.20lint.20error/near/454527007)

Author: adomani

.github/workflows/lint_and_suggest_pr.yml

@@ -21,8 +21,15 @@ jobs:
         with:
           python-version: 3.8
 
+      - name: install elan
+        run: |
+          set -o pipefail
+          curl -sSfL https://github.com/leanprover/elan/releases/download/v3.1.1/elan-x86_64-unknown-linux-gnu.tar.gz | tar xz
+          ./elan-init -y --default-toolchain none
+          echo "$HOME/.elan/bin" >> "${GITHUB_PATH}"
+
       - name: lint
-        continue-on-error: true
+        continue-on-error: true  # allows the following `reviewdog` step to add GitHub suggestions
         run: |
           ./scripts/lint-style.sh --fix
 
@@ -37,7 +44,7 @@ jobs:
           sudo apt-get install -y bibtool
 
       - name: lint references.bib
-        continue-on-error: true
+        continue-on-error: true  # allows the following `reviewdog` step to add GitHub suggestions
         run: |
           ./scripts/lint-bib.sh
 
@@ -65,6 +72,7 @@ jobs:
           echo "$HOME/.elan/bin" >> "${GITHUB_PATH}"
 
       - name: update {Mathlib, Tactic, Counterexamples, Archive}.lean
+        continue-on-error: true  # allows the following `reviewdog` step to add GitHub suggestions
         run: lake exe mk_all
 
       - name: suggester / import list