build: skip vcvarsall if already set up
build: skip vcvarsall if already set up

Without this, running vcbuild repeatedly for some time will cause errors with msbuild saying the command line is too long (due to concatenation happening in vcvarsall).

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso