support/download/svn: use 'svn info' whith LC_ALL=C

Use the 'C' locale when retrieving the date of the last change using
'svn info' since the svn download helper script expect
"Last Changed Date" string.

If another locale is used, the 'date' is empty so the generated
archive (by mk_tar_gz) will not match the expected hash since
the file timestamp is not set properly.

If LANG=fr_FR.UTF-8 is defined in the host system, svn print some
"French encrypted" text:

  eval svn --non-interactive --config-option servers:global:http-timeout=10 info ''\''https://svn.code.sf.net/p/xmlrpc-c/code/advanced@r3176'\'''
  ...
  Date de la dernière modification: 2023-09-02 19:13:35 +0200 (sam. 02 sept. 2023)

diffoscope confirm that the file timestamp is not set correctly
in the generated archive:

$ diffoscope NOK/libxmlrpc-r3176-svn5.tar.gz OK/libxmlrpc-r3176-svn5.tar.gz

Signed-off-by: Julien Olivain <ju.o@free.fr>

support/download/svn

@@ -81,7 +81,7 @@ done
 # In case there is a redirection (e.g. http -> https), just keep the
 # last line (svn outputs everything on stdout)
 # shellcheck disable=SC2086 # creds may be empty
-date="$( _plain_svn info ${creds} "'${uri}@${rev}'" \
+date="$( LC_ALL=C _plain_svn info ${creds} "'${uri}@${rev}'" \
         |sed -r -e '/^Last Changed Date: /!d; s///'
        )"