CI(build-neon): fix accidental neon rebuild on cargo test (#8721)

## Problem

During `Run rust tests` step (for debug builds), we accidentally rebuild
neon twice (by `cargo test --doc` and by `cargo nextest run`).
It happens because we don't set `cov_prefix` for the `cargo test --doc`
command, which triggers rebuilding with different build flags, and one
more rebuild by `cargo nextest run`.

## Summary of changes
- Set `cov_prefix` for `cargo test --doc` to prevent unneeded rebuilds

.github/workflows/_build-and-test-locally.yml

@@ -208,7 +208,7 @@ jobs:
           export LD_LIBRARY_PATH
 
           #nextest does not yet support running doctests
-          cargo test --doc $CARGO_FLAGS $CARGO_FEATURES
+          ${cov_prefix} cargo test --doc $CARGO_FLAGS $CARGO_FEATURES
 
           for io_engine in std-fs tokio-epoll-uring ; do
             NEON_PAGESERVER_UNIT_TEST_VIRTUAL_FILE_IOENGINE=$io_engine ${cov_prefix} cargo nextest run $CARGO_FLAGS $CARGO_FEATURES