Julia: SIGPIPE on Windows builders

[staticfloat]

On the windows buildbots, the NSIS installer maker is reliably throwing a SIGPIPE when run within buildbot. Here's an example log. Unfortunately, this does not occur when the NSIS installer is run outside of the buildbot environment. As best I can tell, the actual way in which subprocesses are invoked from within buildbot is by using twisted.internet.process.Process(), but I have not had time to try to reproduce yet.

[staticfloat]

@vtjnash pointed out this issue, which got me to try running the nsis.exe invocation inside of python then call output.decode('utf-8'), which threw the same error we see in the logs, due to nsis.exe outputting some kind of weird non-UTF-8 symbol, probably for a copyright symbol.

I've switched the default encoding on windows buildbots to ISO-8859-1 which, during manual testing, allowed it to properly decode the nsis.exe output. We'll be able to see if this fixes the windows buildbots after we fix the libgit2 problems. (https://github.com/JuliaLang/julia/issues/24611)

[JeffBezanson]

Wow, because of a copyright symbol, the kingdom was lost :-P

[staticfloat]

This is confirmed fixed. 🙌