Make sure download.test does not fail due to a checksum collision in the download cache

[kit-ty-kate]

Example of random failure: https://github.com/ocaml/opam/actions/runs/13137623611/job/36656730451?pr=6273

 diff --git a/_build/default/tests/reftests/download.test b/_build/default/tests/reftests/download.out
File "tests/reftests/download.test", line 1, characters 0-0:
"C:\Program Files\Git\cmd\git.exe" --no-pager diff --no-index --color=always -u --ignore-cr-at-eol _build/default/tests/reftests/download.test _build/default/tests/reftests/download.out
Command exited with code 1.
index c01c510..e2a08c0 100644
--- a/_build/default/tests/reftests/download.test
+++ b/_build/default/tests/reftests/download.out
@@ -262,7 +262,6 @@ SYSTEM                          mkdir ${OPAMTMP}
 Processing  1/1: [baz.1: http]
 + wget "--header=Accept: */*" "-t" "3" "-O" "${OPAMTMP}/v1.0.0.tar.gz.part" "-U" "opam/current" "--" "https://github.com/UnixJunkie/get_line/archive/v1.0.0.tar.gz"
 SYSTEM                          mv ${OPAMTMP}/v1.0.0.tar.gz.part -> ${OPAMTMP}/v1.0.0.tar.gz
-SYSTEM                          mkdir ${BASEDIR}/OPAM/download-cache/md5-dir/
 SYSTEM                          copy ${OPAMTMP}/v1.0.0.tar.gz -> ${BASEDIR}/OPAM/download-cache/md5-dir/
 SYSTEM                          mkdir ${OPAMTMP}
 Processing  1/1: [baz.1: extract]

This happens because some archives are being created above in the tests and their checksums will vary with the time of day so in some cases a collision will happen with the fix get_line archive