Workaround for Windows Subsystem for Linux bug

On WSL, one user reported that they could not create an archive due to:

tarsnap: link(/root/tarsnap-cache/directory.tmp, /root/tarsnap-cache/directory): Permission denied

I believe [1] that this is due to a known WSL issue [2] which was marked
as fixed 2020-06-02, but that fix is currently only available for
"Insider Preview" builds and not public builds.  Also, I'm not certain
how often WSL users update their installed systems.

[1] http://mail.tarsnap.com/tarsnap-users/msg01601.html

[2] microsoft/WSL#3972

tar/chunks/chunks_directory.c

@@ -528,7 +528,7 @@ chunks_directory_commit(const char * cachepath, const char * osuff,
 
 	/* Create a link from ${cachedir}/directory.tmp. */
 	if (link(t, s)) {
-		if ((errno != ENOSYS) && (errno != EPERM)) {
+		if ((errno != ENOSYS) && (errno != EPERM) && (errno != EACCES)) {
 			warnp("link(%s, %s)", t, s);
 			goto err2;
 		}