Add scripts/build-for-system-tests.sh for local artifact builds

[bm1549]

Description

Adds scripts/build-for-system-tests.sh, a convenience script for building
dd-trace-php tracer artifacts locally for use with system-tests parametric
testing.

Motivation: Running parametric system-tests against a local branch
requires a dd-library-php-<VERSION>-<arch>-linux-gnu.tar.gz bundle and
datadog-setup.php in the system-tests binaries/ directory. Building these
manually from source is error-prone and poorly documented. This script
automates the entire process. It is especially useful on ARM Mac (Apple
Silicon) where the standard CI pipeline is not easily accessible and
cross-compilation is otherwise required.

What the script does:

1. Builds ddtrace.so for PHP 8.2 NTS inside the official
   datadog/dd-trace-ci:php-8.2_buster Docker container (ensuring a correct
   Linux binary regardless of the host OS/arch).
2. Packages the compiled extension and the src/ directory into the
   dd-library-php/trace/ext/<api>/ddtrace.so bundle structure that
   datadog-setup.php and system-tests expect. Profiling and AppSec
   extensions are intentionally omitted (not needed for parametric tests).
3. Writes the output to
   build/packages/dd-library-php-<VERSION>-<arch>-linux-gnu.tar.gz.
4. Optionally copies the tar.gz and datadog-setup.php to a specified
   system-tests binaries/ path via --copy-to <path>.

Usage:

# Build only
./scripts/build-for-system-tests.sh

# Build and copy directly into system-tests
./scripts/build-for-system-tests.sh --copy-to /path/to/system-tests/binaries

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.