chore: respect Cargo parallelism settings for native release builds

[pingzh]

Which issue does this PR close?

No issue filed.

Rationale for this change

The core-amd64-libs and core-arm64-libs targets hardcode cargo build -j 2, limiting compilation to two concurrent jobs regardless of available CPU resources.

This also overrides Cargo's standard CARGO_BUILD_JOBS environment variable and build.jobs configuration, preventing users from controlling parallelism through existing Cargo mechanisms.

Removing the hardcoded limit lets Cargo automatically determine the appropriate level of parallelism while preserving the ability to configure it explicitly:

CARGO_BUILD_JOBS=2 make core-amd64-libs

What changes are included in this PR?

Remove -j 2 from all four Cargo invocations in core-amd64-libs and core-arm64-libs, including optional macOS cross-compilation builds.

How are these changes tested?

Verified the generated commands for both architectures:

make -n core-amd64-libs core-arm64-libs
make -n HAS_OSXCROSS=1 core-amd64-libs core-arm64-libs

Also verified that CARGO_BUILD_JOBS=7 is passed through to all four Cargo invocations using mocked cargo and rustup binaries.

[sunchao]

[P2] Could we forward CARGO_BUILD_JOBS into both release-builder containers before removing this cap? The documented release workflow invokes dev/release/build-release-comet.sh, whose two docker run commands impose --memory 24g and --cpus 6 but do not pass CARGO_BUILD_JOBS or mount the host Cargo configuration. Their entrypoint then runs these Makefile targets, so setting CARGO_BUILD_JOBS=2 on the host does not reach Cargo inside either container. As a result, the actual release build loses its existing two-job limit and cannot use the override described in this PR, while the release profile enables debug information and ThinLTO. The green native CI jobs call cargo build --profile ci directly, so they do not exercise this Dockerized release path. Please propagate the variable through both docker run invocations, ideally with an overridable default of 2.

[pingzh]

addressed via --env "CARGO_BUILD_JOBS=${CARGO_BUILD_JOBS:-2}"

[sunchao]

LGTM

[mbutrovich]

Note that I think this was constrained because we’d occasionally see OOMs in the container with too much parallelism, but we’ll see how it goes for the next release.