Use dotnet msbuild in build.sh so targets can be supplied

This matches what build.ps1 does.

`dotnet build` is an alias for `dotnet msbuild /t:Build`. If a user does
`dotnet build /t:Foo`, dotnet ends up calling `dotnet msbuild /t:Build
/t:Foo`, which may not be what the user expected. See
https://github.com/dotnet/cli/issues/4815 for more information.

So switch to running `dotnet msbuild` so user can supply targets to
`build.sh` and it will just work.

As an example:

    ./build.sh /t:BuildWithoutTesting

Before this patch, this still ends up running `/t:Build`. After this
patch it ends up running `/t:BuildWithoutTesting` as expected, without
any surprises.

build.sh

@@ -77,4 +77,4 @@ export PATH="$DOTNET_INSTALL_DIR:$PATH"
 # Disable first run since we want to control all package sources
 export DOTNET_SKIP_FIRST_TIME_EXPERIENCE=1
 
-dotnet build $REPOROOT/build/build.proj /m:1 /nologo /p:Configuration=$CONFIGURATION /p:Platform="$PLATFORM" "${args[@]}"
+dotnet msbuild $REPOROOT/build/build.proj /m:1 /nologo /p:Configuration=$CONFIGURATION /p:Platform="$PLATFORM" "${args[@]}"