fix: drop support for named pipes on Windows (#962)

Seems that folk are having issues with uploading 0-byte files from
Windows agents. This effectively removes the support for Windows for
uploading from named files that, due to `isFIFO` returning `false` on
Windows for named pipes created using MSYS2's `mkfifo` command, resorted
to checking if the file size is 0 - a common trait of named pipes.

See https://github.com/actions/upload-artifact/issues/281

packages/artifact/__tests__/upload.test.ts

@@ -181,7 +181,10 @@ describe('Upload Tests', () => {
   function hasMkfifo(): boolean {
     try {
       // make sure we drain the stdout
-      return execSync('which mkfifo').toString().length > 0
+      return (
+        process.platform !== 'win32' &&
+        execSync('which mkfifo').toString().length > 0
+      )
     } catch (e) {
       return false
     }