CI(gather-rust-build-stats): fix build with libpq (#8219)

## Problem
I've missed setting `PQ_LIB_DIR` in
#8206 in
`gather-rust-build-stats` job and it fails now:
```
  = note: /usr/bin/ld: cannot find -lpq
          collect2: error: ld returned 1 exit status
          

error: could not compile `storage_controller` (bin "storage_controller") due to 1 previous error
```

https://github.com/neondatabase/neon/actions/runs/9743960062/job/26888597735

## Summary of changes
- Set `PQ_LIB_DIR` for `gather-rust-build-stats` job

.github/workflows/neon_extra_builds.yml

@@ -385,7 +385,7 @@ jobs:
         run: make walproposer-lib -j$(nproc)
 
       - name: Produce the build stats
-        run: cargo build --all --release --timings -j$(nproc)
+        run: PQ_LIB_DIR=$(pwd)/pg_install/v16/lib cargo build --all --release --timings -j$(nproc)
 
       - name: Upload the build stats
         id: upload-stats