use /tmp for mktmpdir
use /tmp for mktmpdir

As we noticed in #4519, we need to use a temporary directory to hold
compact index downloads so that multiple processes don't write to the
same files at the same time and break everything.

The fix for that was #4561, which added temporary directories to hold
all files as they download, and then uses the (atomic) `FileUtils.cp` to
move the completed downloads into place, so there is never a point where
multiple processes are trying to write into the file at once.

Unfortunately, using `Dir.mktmpdir` requires that the parent directory
be _either_ world writable or sticky, but not both. Based on #4599, it
looks like it's common for home directories to be both world writable
and sticky. While that's a security problem by itself, it's not a big
concern for Bundler and the compact index. So we want to let users
continue to use Bundler, even with the compact index, without having to
change the permissions on their home directories.

This commit changes the `mktmpdir` call to create the temporary
directory inside the default OS tempdir, which is typically `/tmp` or
`/var/tmp` depending on distro. Since that directory is designed to hold
other temporary directories, that change should (theoretically) reduce
or eliminate the problem reported in #4599.

bundler/bundler,mvz/bundler,dilumnavanjana/bundler,dilumnavanjana/bundler,mvz/bundler,bundler/bundler