make-single-file-spm: better directory name (#1446)
make-single-file-spm: better directory name (#1446)

Motivation:

The make-single-file-spm script used to use a hidden folder in /tmp
which isn't very helpful when tar'ing up the result for a bug report.

Modifications:

Switch to a non-hidden folder.

Result:

Easier bugs.swift.org reports.
apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio,apple/swift-nio