fix: use pre-installed Amazon.Lambda.Tools for dotnet6 builds

[ckawl]

Issue #, if available:

Description of changes

Amazon.Lambda.Tools 7.0.0 dropped support for .NET 6 (net6.0), only supporting net8.0 and net10.0. The
GlobalToolInstall action always installs/updates the tool to latest at build time, so dotnet6 container builds (sam
build --use-container) fail with NU1202 — even though the SAM build image already ships a working, net6-compatible
version of the tool (currently 6.0.6 at /var/lang/bin).

This change makes GlobalToolInstallAction runtime-aware, for dotnet6 only:

• Probe for a working tool first (dotnet lambda help). If one is available (e.g. pre-installed in the SAM build
  image), skip the install/update and use it as is.
• If no tool is available (e.g. local builds outside the build image), fall back to the existing install behavior
  unchanged.
• The skip deliberately does not set the process-wide __tools_installed flag, so other runtimes in the same process
  still install/update to latest exactly as before.

All other runtimes are unaffected — the probe only runs for dotnet6.

Version-pinning approaches were considered and rejected: NuGet refuses in-place downgrades (tool update --version X
fails when a higher version is installed, and --allow-downgrade only suppresses the warning), and a hardcoded version
in lambda-builders would duplicate ownership of the tool version that belongs to the build image.

Description of how you validated changes

• Unit tests added: dotnet6 skips when a tool is pre-installed; dotnet6 falls back to install when absent; other
  runtimes never probe; a dotnet8 build after a dotnet6 skip still installs/updates as before. Full unit suite passes
  (810 tests).
• End-to-end validated on x86_64: applied this patch to the lambda-builders venv inside the published
  public.ecr.aws/sam/build-dotnet6:latest-x86_64 image, then ran sam build --use-container on a sam init dotnet6
  hello-world app:
  • Unpatched published image: fails with NU1202 (tool update pulls 7.0.0)
  • Patched image: probe detects the pre-installed Amazon.Lambda.Tools 6.0.6, logs "Skipping Amazon.Lambda.Tools
    install/update for dotnet6; using the pre-installed version", and the build succeeds.

Checklist

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