Update 'mktemp' invocation

Turns out using "-t TEMPLATE" instead of "TEMPLATE" is causing TMPDIR to
override the setting from `-p`, which is the opposite of what the man
page says it does.

optipdf

@@ -257,7 +257,7 @@ ERRORS=0
 FREED_TOTAL=0
 for FILE in "${FILES[@]}"; do
     setup_tempdir
-    TEMP=$(mktemp -p "$TEMP_DIR" -t 'file.XXXXXX')
+    TEMP=$(mktemp -p "$TEMP_DIR" 'file.XXXXXX')
     rm -f -- "$TEMP"
     BEGIN_FILESIZE=$(file_size "$FILE")