[ci] Install maui-android workload on Linux for Android test pipeline

[Copilot]

Description of Change

The Android test pipeline now runs on Linux infrastructure (AndroidPoolLinux), but the workload installation step was attempting to install the full maui workload, which includes iOS/macCatalyst components that cannot be installed on Linux, causing build failures.

Changes:

• Modified src/DotNet/DotNet.csproj Install target to conditionally install workloads based on OS:
  • Linux: Install maui-android workload only
  • macOS/Windows: Install full maui workload
• Follows existing pattern from _InstallWorkloadPacks target (lines 186-189) which already excludes iOS/macCatalyst/macOS workloads on Linux

Example:

<!-- Before: Always installed full maui workload -->
<_LocalWorkloadIds Include="maui" />

<!-- After: Platform-specific workload installation -->
<_LocalWorkloadIds Include="maui-android" Condition="$([MSBuild]::IsOSPlatform('linux'))" />
<_LocalWorkloadIds Include="maui" Condition="!$([MSBuild]::IsOSPlatform('linux'))" />

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