[CI] Save sccache logs (#155444)

This patch saves the sccache logs to the artifacts. If sccache dies and
the server prints logs, we currently do not collect them anywhere and
they do not get dumped to STDOUT/STDERR. If the process is directly
getting killed (SIGTERM), it seems like it doesn't dump anything, but in
most other cases we should be able to see something.

Related to #155442.

.github/workflows/premerge.yaml

diff --git a/.github/workflows/premerge.yaml b/.github/workflows/premerge.yaml
index 9d92551..2b43f46 100644
--- a/.github/workflows/premerge.yaml
+++ b/.github/workflows/premerge.yaml
@@ -65,7 +65,7 @@
           # several test suites in a row and discard statistics that we want
           # to save in the end.
           export SCCACHE_IDLE_TIMEOUT=0
-          sccache --start-server
+          SCCACHE_LOG=info SCCACHE_ERROR_LOG=artifacts/sccache.log sccache --start-server
 
           ./.ci/monolithic-linux.sh "${projects_to_build}" "${project_check_targets}" "${runtimes_to_build}" "${runtimes_check_targets}" "${runtimes_check_targets_needs_reconfig}" "${enable_cir}"
       - name: Upload Artifacts
@@ -117,7 +117,7 @@
           call C:\\BuildTools\\Common7\\Tools\\VsDevCmd.bat -arch=amd64 -host_arch=amd64
           # See the comments above in the Linux job for why we define each of
           # these environment variables.
-          bash -c "export SCCACHE_GCS_BUCKET=$CACHE_GCS_BUCKET; export SCCACHE_GCS_RW_MODE=READ_WRITE; export SCCACHE_IDLE_TIMEOUT=0; sccache --start-server; .ci/monolithic-windows.sh \"${{ steps.vars.outputs.windows-projects }}\" \"${{ steps.vars.outputs.windows-check-targets }}\""
+          bash -c "export SCCACHE_GCS_BUCKET=$CACHE_GCS_BUCKET; export SCCACHE_GCS_RW_MODE=READ_WRITE; export SCCACHE_IDLE_TIMEOUT=0; SCCACHE_LOG=info SCCACHE_ERROR_LOG=artifacts/sccache.log sccache --start-server; .ci/monolithic-windows.sh \"${{ steps.vars.outputs.windows-projects }}\" \"${{ steps.vars.outputs.windows-check-targets }}\""
       - name: Upload Artifacts
         # In some cases, Github will fail to upload the artifact. We want to
         # continue anyways as a failed artifact upload is an infra failure, not