[wasm] Make sure the TargetOS is capitalized (#54927)

[wasm] Make sure the TargetOS is capitalized (#54927)

Fixes: https://github.com/dotnet/runtime/issues/54926

The $(TargetOS) is used in $(TargetFrameworks) and is case sensitive.
Make sure it is capitalized for Browser, so that build.cmd works when
used like:

    ./build.cmd -bl -os browser -subset libs

It fixes build errors like:

    C:\r\src\libraries\shims\manual\System.forwards.cs(8,88): error CS0234: The type or namespace name 'ZLibException' does not exist in the namespace 'System.IO.Compression' (are you missing an assembly reference?) [C:\r\src\libraries\shims\manual\System.csproj]
    C:\r\src\libraries\shims\manual\System.forwards.cs(9,77): error CS0234: The type or namespace name 'CookieVariant' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [C:\r\src\libraries\shims\manual\System.csproj]
    C:\r\src\libraries\shims\manual\System.forwards.cs(10,77): error CS0234: The type or namespace name 'PathList' does not exist in the namespace 'System.Net' (are you missing an assembly reference?) [C:\r\src\libraries\shims\manual\System.csproj]