Blindly try to fix test script's tar invocation for MSYS.

Buildfarm member fairywren doesn't like the test case I added
in commit 081876d75.  I'm guessing the reason is that I shouldn't
be using a perl2host-ified path in the tar command line.

src/bin/pg_basebackup/t/010_pg_basebackup.pl

@@ -270,9 +270,10 @@ SKIP:
 	$node2->init_from_backup($node, 'tarbackup2', tar_program => $tar);
 
 	# Recover tablespace into a new directory (not where it was!)
-	mkdir "$tempdir/tblspc1replica";
+	my $repTsDir     = "$tempdir/tblspc1replica";
 	my $realRepTsDir = TestLib::perl2host("$shorter_tempdir/tblspc1replica");
-	TestLib::system_or_bail($tar, 'xf', $tblspc_tars[0], '-C', $realRepTsDir);
+	mkdir $repTsDir;
+	TestLib::system_or_bail($tar, 'xf', $tblspc_tars[0], '-C', $repTsDir);
 
 	# Update tablespace map to point to new directory.
 	# XXX Ideally pg_basebackup would handle this.