scripts: make Build-Pack and Deploy-Brass cross-platform

Two small fixes that surface when running the deploy pipeline on a
non-Windows host (we now do this on glados via the Telegram bridge):

- Build-Pack.ps1: fall back to launcher/ModpackLauncher.csproj <Version>
  when Get-Item.VersionInfo.FileVersion returns empty. Linux PowerShell
  can't parse PE FileVersion from a Windows .exe, so the previous code
  threw "set <Version> in ModpackLauncher.csproj and republish" -- but
  the csproj <Version> *was* set, just not readable from a foreign PE.
  Appends ".0" so the embedded value still matches the 4-part form the
  Windows compile bakes in.

- Deploy-Brass.ps1: prefer rsync where available, fall back to robocopy
  otherwise. robocopy is Windows-only and bails on Linux/macOS hosts.

scripts/Deploy-Brass.ps1

@@ -214,11 +214,26 @@ $overridesLocal = Join-Path $repoRoot 'pack\overrides'
 $shareFiles     = Join-Path $DeployShare 'files'
 if ($shouldRunPack -and (Test-Path $overridesLocal)) {
     Step "Mirror pack/overrides/ -> $shareFiles" {
-        # /MIR makes destination match source (deletes orphan files in $shareFiles).
-        # /XJ skips junctions, /R:1 /W:1 keeps retry behaviour sane on flaky shares.
-        robocopy $overridesLocal $shareFiles /MIR /XJ /R:1 /W:1 /NFL /NDL /NJH /NJS /NP | Out-Host
-        # Robocopy returns 0-7 for success-with-info; 8+ is real failure.
-        if ($LASTEXITCODE -ge 8) { throw "robocopy failed with exit $LASTEXITCODE" }
+        # Prefer rsync where available (Linux/macOS); fall back to Windows robocopy.
+        # Both perform a mirror: orphan files in the destination are removed so
+        # the share matches the local overrides exactly.
+        if (Get-Command rsync -ErrorAction SilentlyContinue) {
+            $src = ($overridesLocal -replace '\\','/').TrimEnd('/') + '/'
+            $dst = ($shareFiles    -replace '\\','/').TrimEnd('/') + '/'
+            if (-not (Test-Path $shareFiles)) { New-Item -ItemType Directory -Path $shareFiles -Force | Out-Null }
+            & rsync -a --delete $src $dst
+            if ($LASTEXITCODE -ne 0) { throw "rsync failed with exit $LASTEXITCODE" }
+        }
+        elseif (Get-Command robocopy -ErrorAction SilentlyContinue) {
+            # /MIR makes destination match source (deletes orphan files in $shareFiles).
+            # /XJ skips junctions, /R:1 /W:1 keeps retry behaviour sane on flaky shares.
+            robocopy $overridesLocal $shareFiles /MIR /XJ /R:1 /W:1 /NFL /NDL /NJH /NJS /NP | Out-Host
+            # Robocopy returns 0-7 for success-with-info; 8+ is real failure.
+            if ($LASTEXITCODE -ge 8) { throw "robocopy failed with exit $LASTEXITCODE" }
+        }
+        else {
+            throw "Neither rsync nor robocopy is available -- can't mirror pack overrides."
+        }
     }
 }