CI(neon-image): add ARM-specific RUSTFLAGS (#8566)

## Problem

It's recommended that a couple of additional RUSTFLAGS be set up to
improve the performance of Rust applications on AWS Graviton.

See
https://github.com/aws/aws-graviton-getting-started/blob/57dc813626d0266f1cc12ef83474745bb1f31fb4/rust.md

Note: Apple Silicon is compatible with neoverse-n1:
```
$ clang --version
Apple clang version 15.0.0 (clang-1500.3.9.4)
Target: arm64-apple-darwin23.6.0
Thread model: posix
InstalledDir: /Applications/Xcode_15.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
$
$ clang --print-supported-cpus 2>&1 | grep neoverse-
	neoverse-512tvb
	neoverse-e1
	neoverse-n1
	neoverse-n2
	neoverse-v1
	neoverse-v2
```

## Summary of changes
- Add `-Ctarget-feature=+lse -Ctarget-cpu=neoverse-n1` to RUSTFLAGS for
ARM images

.github/workflows/build_and_test.yml

@@ -503,7 +503,10 @@ jobs:
       - uses: docker/build-push-action@v6
         with:
           context: .
+          # ARM-specific flags are recommended for Graviton ≥ 2, these flags are also supported by Ampere Altra (Azure)
+          # https://github.com/aws/aws-graviton-getting-started/blob/57dc813626d0266f1cc12ef83474745bb1f31fb4/rust.md
           build-args: |
+            ADDITIONAL_RUSTFLAGS=${{ matrix.arch == 'arm64' && '-Ctarget-feature=+lse -Ctarget-cpu=neoverse-n1' || '' }}
             GIT_VERSION=${{ github.event.pull_request.head.sha || github.sha }}
             BUILD_TAG=${{ needs.tag.outputs.build-tag }}
             TAG=${{ needs.build-build-tools-image.outputs.image-tag }}

Dockerfile

@@ -35,8 +35,9 @@ COPY --from=pg-build /home/nonroot/pg_install/v16/include/postgresql/server pg_i
 COPY --from=pg-build /home/nonroot/pg_install/v16/lib                       pg_install/v16/lib
 COPY --chown=nonroot . .
 
+ARG ADDITIONAL_RUSTFLAGS
 RUN set -e \
-    && PQ_LIB_DIR=$(pwd)/pg_install/v16/lib RUSTFLAGS="-Clinker=clang -Clink-arg=-fuse-ld=mold -Clink-arg=-Wl,--no-rosegment" cargo build \
+    && PQ_LIB_DIR=$(pwd)/pg_install/v16/lib RUSTFLAGS="-Clinker=clang -Clink-arg=-fuse-ld=mold -Clink-arg=-Wl,--no-rosegment ${ADDITIONAL_RUSTFLAGS}" cargo build \
       --bin pg_sni_router  \
       --bin pageserver  \
       --bin pagectl  \