By default, unzip will attempt to query the user when files to be
installed already exist. Unfortunately, if the install script is run
with `sh -s`, unzip will read the remaining portion of the script as
input, resulting in a mess on the terminal.
This commit changes the unzip behavior to clobber existing files: this
already happens for platforms using tar, so it's not hugely breaking
(and I could find no evidence that we believe this is more likely to
cause issues on Windows)
ef967271e6