cmd/cue: fix up xml+koala testscript to actually test stdin

As can be seen by the stdin line just above, I intended to feed
standard input to the `cue export` line, but I messed up a copy-paste.
This error wasn't spotted as not reading stdin is not an error.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ib8d54f368c5b877ab96ceb6096ddf6ba9aa94933
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1214304
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Paul Jolly <[email protected]>

Author: mvdan

cmd/cue/cmd/testdata/script/encoding_xml_koala.txtar

@@ -26,7 +26,7 @@ cmp stdout export.json
 exec cue export xml+koala: export.xml
 cmp stdout export.json
 stdin export.xml
-exec cue export xml+koala: export.xml
+exec cue export xml+koala: -
 cmp stdout export.json
 
 exec cue import -o - xml+koala: export.xml