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__/test-artifact-file.sh

@@ -18,8 +18,10 @@ if [ ! -f "$path" ]; then
     exit 1
 fi
 
-actualContent=$(cat $path)
-if [ "$actualContent" != "$expectedContent" ];then
+actualContent=$(cat "$path")
+if [ "$expectedContent" == "_EMPTY_" ] && [ ! -s "$path" ]; then
+    exit 0
+elif  [ "$actualContent" != "$expectedContent" ]; then
     echo "File contents are not correct, expected $expectedContent, received $actualContent"
     exit 1
 fi

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
     }