build tools: use 64-bit msbuild if present

This makes build.bat behavior consistent with that of test.cmd with
respect to bitness of PowerShell used to run tests.

Otherwise, test.cmd would run tests using 64-bit PowerShell, while
build.bat would run them using 32-bit PowerShell (via 32-bit msbuild).
build tools: fix building the nupkg with non-default Chocolatey install path

Replace hardcoded %systemdrive%\Chocolatey with %ChocolateyInstall%.
Merge pull request #479 from jberezanski/build-fixes

Build script fixes: hardcoded Chocolatey directory, use 64-bit msbuild