fetcher: give tmpfile ability to create

When forcing the linux fallback (no O_TMPFILE) it triggers an error:
encountered errors: openat fetcher.xxxxxxxx: no such file or directory,
when adding the O_CREATE flag the fallback's error was fixed. This also
adds the flags to the unix and windows variants.

Signed-off-by: crozzy <joseph.crosland@gmail.com>

Author: crozzy

libindex/tempfile_linux.go

@@ -46,6 +46,7 @@ Loop:
 		flag := os.O_WRONLY
 		if !tmpOK {
 			name = fetchFilename()
+			flag |= os.O_CREATE | os.O_EXCL // Create new file, fail if exists
 		} else {
 			flag |= unix.O_TMPFILE
 		}

libindex/tempfile_unix.go

@@ -10,7 +10,7 @@ import (
 )
 
 func openTemp(dir *os.Root) (f *os.File, err error) {
-	const flag = os.O_WRONLY
+	const flag = os.O_WRONLY | os.O_CREATE | os.O_EXCL
 	for {
 		name := fetchFilename()
 		f, err = dir.OpenFile(name, flag, 0o644)

libindex/tempfile_windows.go

@@ -12,7 +12,7 @@ import (
 func openTemp(dir *os.Root) (f *os.File, err error) {
 	// Copied out of golang.org/x/sys/windows
 	const FILE_FLAG_DELETE_ON_CLOSE = 0x04000000
-	const flag = os.O_WRONLY | FILE_FLAG_DELETE_ON_CLOSE
+	const flag = os.O_WRONLY | os.O_CREATE | os.O_EXCL | FILE_FLAG_DELETE_ON_CLOSE
 	for {
 		name := fetchFilename()
 		f, err = dir.OpenFile(name, flag, 0o644)