[#125] Build and test on Linux

[SkowronskiAndrew]

Summary

Fixes #125.

The GitHub Actions matrices only covered Windows and Mac, even though the Unity Editor also runs on
Linux and the repo has always bundled a Linux UnityFileSystemApi.so. This adds a Linux leg to both
workflows, so the test suite now runs on Linux for every PR (guarding against cross-platform
regressions) and the build action produces a Linux artifact that can be attached to future releases.

No product code changes were needed — the .so was already wired up as a CopyToOutputDirectory
item in UnityFileSystem.csproj, DllImport("UnityFileSystemApi") resolves it via .NET's default
probing on Linux, and there is no platform-conditional code in the tool.

Linux is x64 only. The Unity Editor for Linux ships x86_64 only, and the issue says our
configuration can follow what the Editor supports; there is also no arm64 .so to pair with an
arm64 runner.

Changes

Workflows

• test.yml and .build.yml: added a linux / x64 / ubuntu-latest entry to both matrices.
• Both matrices switch from os × arch plus an exclude list to explicit include entries.
  This was needed because GitHub's Linux runner is ubuntu-latest, so runs-on can no longer be
  derived by appending -latest to the platform name. Keeping os: linux (rather than ubuntu)
  means artifact and release-zip names stay platform-flavoured: UnityDataTool-linux-x64-release.
• .build.yml: the two near-duplicate per-OS cleanup steps (one pwsh, one bash) collapse into a
  single bash step. Each matrix entry now names the nativelib its package keeps, and the step
  deletes the other two plus any .pdb.

Docs

• README.md: the Downloads section said prebuilt Windows and Mac builds are published; now
  Windows, Mac, and Linux.

Testing

• Ran tests locally (Windows)
• Triggered the build and test jobs on this branch and confirmed linux builds and tests all passed
• Downloaded the linux build and confirm zip file contains expected contents.

todo: a validation on a linux machine that the binary works.

Agent Pre-flight checks: no OSPlatform/RuntimeInformation branching in the
tool's own code, no hardcoded backslash paths in the C# sources, no embedded Windows paths in the
ExpectedData golden files, and .gitattributes already normalises text to LF on all platforms.

🤖 Generated with Claude Code

[bastienunity]

tested on linux (Ubuntu 26.04). I didn't had any valid serialized file for the Unity version it support but I got the proper errors for unsupported serialization format version when trying to de-serialize a content file.