faster disk cleanup check (#8565)

I can't reproduce the `chmod` failure and have no clue how to fix it, so
for now let's just hope it's not too frequent.

CHANGELOG_BEGIN
CHANGELOG_END

ci/build-unix.yml

@@ -33,27 +33,19 @@ steps:
       # Location of the disk cache for CI servers set in their init files:
       # infra/macos/2-common-box/init.sh:echo "build:darwin --disk_cache=~/.bazel-cache" > ~/.bazelrc
       # infra/vsts_agent_linux_startup.sh:echo "build:linux --disk_cache=~/.bazel-cache" > ~/.bazelrc
-      disk_cache=$HOME/.bazel-cache
-      if [ $(du -ms "$disk_cache" | awk '{print $1}') -gt 80000 ]; then
-          echo "Deleting '$disk_cache'..."
+      if [ $(df -m . | sed 1d | awk '{print $4}') -lt 30000 ]; then
+          echo "Disk full, cleaning up..."
+          disk_cache="$HOME/.bazel-cache"
           rm -rf "$disk_cache"
-      else
-          echo "Disk cache small enough:"
-          du -hs "$disk_cache"
-      fi
-      local_cache=$HOME/.cache/bazel
-      if [ -d "$local_cache" ]; then
-          if [ $(du -ms "$local_cache" | awk '{print $1}') -gt 80000 ]; then
-              echo "Deleting '$local_cache'..."
+          echo "removed '$disk_cache'"
+          local_cache="$HOME/.cache/bazel"
+          if [ -d "$local_cache" ]; then
               chmod -R +w "$local_cache"
               rm -rf "$local_cache"
-          else
-              echo "Local cache small enough:"
-              du -hs "$local_cache"
+              echo "removed '$local_cache'"
           fi
-      else
-          echo "No '$local_cache', moving on."
       fi
+      df -h .
     displayName: clean-up disk cache
 
   - bash: ./fmt.sh --test