Invoking msbuild with runInShell makes handling path escaping more
error-prone, and substantially increases the chances of running into
maximum path limits. This replaces the direct call with a .bat wrapper
that calls vsvars64.bat then msbuild, and uses relative paths within the
script to keep command lengths short.
Fixes https://github.com/flutter/flutter/issues/32792
Eliminates the need for a build.bat in the Windows build workflow, adding
preliminary support for building using msbuild. The handling of
vcvars64.bat may be refined in the future, but this serves as a starting point.
