workflows: use ubuntu-slim for actions

Actions here are rather trivial, can be handled by slim runner.
Follow https://github.com/nspcc-dev/.github/blob/6c4dff04862ad947805e82fb9b664899906c6305/gh.md

Signed-off-by: Roman Khimov <roman@nspcc.ru>

Author: roman-khimov

.github/workflows/buf.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   lint:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     steps:
       - uses: actions/checkout@v4
       - name: Download buf
@@ -21,7 +21,7 @@ jobs:
       - run: ./buf-Linux-x86_64 lint
 
   breaking:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     steps:
       - name: Download buf
         uses: dsaltares/fetch-gh-release-asset@1.1.2